xfs: check da node magic in _node_lookup_int

Before we start processing what we /think/ is a da3 node block, actually
check the magic to make sure that we're looking at a node block.  This
way we won't blow the asserts in _node_hdr_from_disk on corrupted
metadata.
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>

fs/xfs/libxfs/xfs_da_btree.c

@@ -1523,8 +1523,10 @@ xfs_da3_node_lookup_int(
 			break;
 		}
 
-		blk->magic = XFS_DA_NODE_MAGIC;
+		if (magic != XFS_DA_NODE_MAGIC && magic != XFS_DA3_NODE_MAGIC)
+			return -EFSCORRUPTED;
 
+		blk->magic = XFS_DA_NODE_MAGIC;
 
 		/*
 		 * Search an intermediate node for a match.