net: netrom: Add missing annotation for nr_neigh_start()

Sparse reports a warning at nr_neigh_start()
warning: context imbalance in nr_neigh_start() - wrong count at exit
The root cause is the missing annotation at nr_neigh_start()
Add the missing __acquires(&nr_neigh_list_lock) annotation

Signed-off-by: Jules Irenge <jbi.octave@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>