find_node: The reference count needs to be increased while the lock
is held. Otherwise, the node may disappear right after the lock was
released and increase was attempted, leading to an oops.
remove_node: The reference count needs to be checked while the lock
is held. Otherwise, the node may be used in the match function or
returned from find_node while it has a zero refcount.
Jan Engelhardt
93c7d0ac47