Line up colons across read, write, and trim thread stats

Line up colons across read, write, and trim thread stats

Line up the colons across the read, write, and trim
thread stats rows.

Example:
drive_0: (groupid=0, jobs=576): err= 0: pid=42198: Wed Dec 21 19:51:35 2016
  read : io=420642MB, bw=33953MB/s, iops=8289.3K, runt= 12389msec
  write: io=325276MB, bw=26177MB/s, iops=6390.9K, runt= 12426msec
  trim: io=0MB, bw=0MB/s, iops=0, runt= 12426msec

becomes:
drive_0: (groupid=0, jobs=576): err= 0: pid=42198: Wed Dec 21 19:51:35 2016
   read: io=420642MB, bw=33953MB/s, iops=8289.3K, runt= 12389msec
  write: io=325276MB, bw=26177MB/s, iops=6390.9K, runt= 12426msec
   trim: io=0MB, bw=0MB/s, iops=0, runt= 12426msec

Signed-off-by: Jens Axboe <axboe@fb.com>