xfs: consolidate btree ptr checking

Merge xfs_btree_check_sptr and xfs_btree_check_lptr into a single
__xfs_btree_check_ptr that can be shared between xfs_btree_check_ptr
and the scrub code.

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