xfs: convert xfs_fs_geometry to use mount feature checks

Reporting filesystem features to userspace is currently superblock
based. Now we have a general mount-based feature infrastructure,
switch to using the xfs_mount rather than the superblock directly.

This reduces the size of the function by over 300 bytes.

$ size -t fs/xfs/built-in.a
	text    data     bss     dec     hex filename
before	1127855  311352     484 1439691  15f7cb (TOTALS)
after	1127535  311352     484 1439371  15f68b (TOTALS)

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>