Btrfs: drop WARN_ON from btrfs_add_leaf_ref

btrfs_add_leaf_ref was doing checks on the objects it found in the
rbtree to make sure they were properly linked into the tree.  But, the field
it was checking can be safely changed outside of the tree spin lock.

The WARN_ON was for debugging the initial implementation and can be
safely removed.

Signed-off-by: Chris Mason <chris.mason@oracle.com>