xfs: rename xfs_buftarg structure members

In preparation for adding new members to the structure,
give these old ones more descriptive names:

	bt_ssize -> bt_meta_sectorsize
	bt_smask -> bt_meta_sectormask

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Ben Myers <bpm@sgi.com>