fstests: fix fallocate commands that need the unshare switch

fstests: fix fallocate commands that need the unshare switch

We re-added the UNSHARE flag to fallocate, so go make sure that all
the unshare tests actually check that the installed copy of xfs_io
supports the 'funshare' command and that the underlying filesystem
understands the flag, and change the tests themselves to use
funshare.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eryu Guan <eguan@redhat.com>