LU-15459 llite: clear async errors on write commit sync

LU-15459 llite: clear async errors on write commit sync

Async errors should be cleared after vvp_io_commit_sync(). Otherwise,
that will be done in ll_flush() called from
linux/fs/open.c:filp_close() and close(2) will fail. ll_flush()
replaces any error code with EIO which is confusing.

Test to illustrate the issue is added.
'P' mode is added to multiop. It is like 'w' but does only 1 write
call regardless to how many bytes were written.

HPE-bug-id: LUS-7529
Signed-off-by: Vladimir Saveliev <vlaidimir.saveliev@hpe.com>
Change-Id: I6b7a1465268999b48a50f3584f3821f4b088303d
Reviewed-on: https://review.whamcloud.com/46178
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Alexander Boyko <alexander.boyko@hpe.com>
Reviewed-by: Andrew Perepechko <andrew.perepechko@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>