xfs: factor out a __xfs_btree_check_lblock_hdr helper

This will allow sharing code with the in-memory block checking helper.

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>