fstests: run xfs_io with -i option if supported

fstests: run xfs_io with -i option if supported

Try to run xfs_io with command line option -i, which starts an idle
thread before performing any io.

The purpose of this idle thread is to test io from a multi threaded
process. With single threaded process, the file table is not shared
and file structs are not reference counted.

In order to improve the chance of detecting file struct reference
leaks, we should run xfs_io commands with this option as much as
possible.

Analysis of the effect of xfs_io -i on tests runtime showed that it
may lead to slightly longer run times in extreme cases (e.g +3s for
generic/132), but has a negligible effect on runtime of tests among
the 'quick' group (worst case +0.3s for generic/130).

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>