engines/sg: improve error handling

engines/sg: improve error handling

The Linux sg driver accepts only 16 SCSI commands in flight
at a time per file descriptor. fio does not exit gracefully
when it attempts to queue up 17 or more trim commands via
the sg ioengine. This patch improves error handling in the
sg ioengine commit function to achieve a graceful exit if
fio attempts to queue up too many trim commands. The key to
this is calling clear_io_u on each io_u in the 17th trim
command. With this patch fio no longer loops forever
waiting for the IOs in the (not succesffully submitted)
17th trim command to complete.

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