xfs: add a xfs_btree_ptrs_equal helper

This only has a single caller and thus might be a bit questionable,
but I think it really improves the readability of
xfs_btree_visit_block.

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>