debug: make debug=io readable with multiple threads

debug: make debug=io readable with multiple threads

When multiple threads are active, debug=io prints are unreadable as the output
is interleaved. Multiple log_info_buf() calls are used to construct each line,
but there is no mutual exclusion covering multiple calls.

Change the dprint call tree to construct the entire line before passing it to
log_info_buf(), rather than make several calls.

Other nits:
* print the thread ID rather than the process ID
* change offset and length from decimal to hex
* separate offset, length, ddir, and file with , rather than / since the
  filename on the same line likely has / of its own
* change "fill_io_u" to "fill" to match the others
* change "io complete" to "complete" to match the others
* change "->prep()=%d" to "prep: io_u %p: ret=%d" to resemble the others
* change offset/buflen in an error print to better resemble the normal prints
* add "file=" prefix for the filename
* check the calloc() return values inside the valist_to_buf functions

Old:
fill_io_u: io_u 0x7feeac010b80: off=720896/len=65536/ddir=1

io       50692io       50692//dev/dax0.0io       50692fill_io_u: io_u 0x7fee98010b80: off=196608/len=65536/ddir=1io       50692io       50692
io       50692->prep(0x7fef10010b80)=0
//dev/dax0.0//dev/dax1.0io       50692io       50692io       50692io       50692->prep(0x7feeec010b80)=0
io       50692prep: io_u 0x7feec4010b80: off=1966080/len=65536/ddir=1io       50692io complete: io_u 0x7feedc010b80: off=393216/len=65536/ddir=1io
50692io       50692prep: io_u 0x7feef4010b80: off=720896/len=65536/ddir=1io       50692io       50692
io       50692io       50692prep: io_u 0x7feef0010b80: off=851968/len=65536/ddir=1//dev/dax0.0io       50692//dev/dax0.0

//dev/dax0.0
io       50692io       50692//dev/dax0.0//dev/dax0.0io       50692

New:
io       71400 queue: io_u 0x7fd0f0010b80: off=0x2f0000,len=0x10000,ddir=1,file=/dev/dax0.0
io       71395 fill: io_u 0x7fd0fc010b80: off=0x80000,len=0x10000,ddir=1,file=/dev/dax0.0
io       71395 prep: io_u 0x7fd0fc010b80: off=0x80000,len=0x10000,ddir=1,file=/dev/dax0.0
io       71395 prep: io_u 0x7fd0fc010b80: ret=0
io       71395 queue: io_u 0x7fd0fc010b80: off=0x80000,len=0x10000,ddir=1,file=/dev/dax0.0
io       71430 complete: io_u 0x7fd05c010b80: off=0x180000,len=0x10000,ddir=1,file=/dev/dax1.0
io       71400 complete: io_u 0x7fd0f0010b80: off=0x2f0000,len=0x10000,ddir=1,file=/dev/dax0.0
io       71430 fill: io_u 0x7fd05c010b80: off=0x190000,len=0x10000,ddir=1,file=/dev/dax1.0
io       71400 fill: io_u 0x7fd0f0010b80: off=0x300000,len=0x10000,ddir=1,file=/dev/dax0.0
io       71400 prep: io_u 0x7fd0f0010b80: off=0x300000,len=0x10000,ddir=1,file=/dev/dax0.0
io       71400 prep: io_u 0x7fd0f0010b80: ret=0
io       71400 queue: io_u 0x7fd0f0010b80: off=0x300000,len=0x10000,ddir=1,file=/dev/dax0.0
io       71430 prep: io_u 0x7fd05c010b80: off=0x190000,len=0x10000,ddir=1,file=/dev/dax1.0
io       71395 complete: io_u 0x7fd0fc010b80: off=0x80000,len=0x10000,ddir=1,file=/dev/dax0.0
io       71430 prep: io_u 0x7fd05c010b80: ret=0
io       71430 queue: io_u 0x7fd05c010b80: off=0x190000,len=0x10000,ddir=1,file=/dev/dax1.0
io       71421 complete: io_u 0x7fd090010b80: off=0x320000,len=0x10000,ddir=1,file=/dev/dax0.0
io       71419 complete: io_u 0x7fd098010b80: off=0x320000,len=0x10000,ddir=1,file=/dev/dax0.0
io       71404 complete: io_u 0x7fd0d0010b80: off=0x0,len=0x10000,ddir=1,file=/dev/dax0.0

Signed-off-by: Jens Axboe <axboe@kernel.dk>