Change xfs_sb_from_disk() interface to take a mount pointer
instead of a superblock pointer.

This is to print mount point specific error messages in future
fixes.
Signed-off-by: NChandra Seetharaman <sekharan@us.ibm.com>
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NBen Myers <bpm@sgi.com>