fstests: always use fail-at-unmount semantics for XFS

fstests: always use fail-at-unmount semantics for XFS

Rather than require every test that tests unmount in failure
conditions have to set up fail-at-unmount semantics for the
underlying filesystem, use these semantics for all test and scratch
device mounts.

This currently only affects XFS filesystems, and helps prevent
unexpected unmount hangs in EIO tests because metadata writes are
configured to try forever by default.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Zorro lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>