fsck.xfs: allow forced repairs using xfs_repair

The fsck.xfs script did nothing, because xfs doesn't need a fsck to be
run on every unclean shutdown. However, sometimes it may happen that the
root filesystem really requires the usage of xfs_repair and then it is a
hassle. This patch makes the situation a bit easier by detecting forced
checks (/forcefsck or fsck.mode=force) and invoking xfs_repair.

Signed-off-by: Jan Tulak <jtulak@redhat.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>