common/rc: Add a helper to run fsx on a given file

common/rc: Add a helper to run fsx on a given file

Currently run_fsx is hardcoded to run on a file in $TEST_DIR.
Add a helper _run_fsx_on_file so that we can run fsx on any
given file including in $SCRATCH_MNT. Also, refactor _run_fsx
to use this helper.

No functional change is intended in this patch.

Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: John Garry <john.g.garry@oracle.com>
Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>