netlink: eliminate nl_sk_hash_lock

netlink: eliminate nl_sk_hash_lock

As rhashtable_lookup_compare_insert() can guarantee the process
of search and insertion is atomic, it's safe to eliminate the
nl_sk_hash_lock. After this, object insertion or removal will
be protected with per bucket lock on write side while object
lookup is guarded with rcu read lock on read side.

Signed-off-by: Ying Xue <[email protected]>
Cc: Thomas Graf <[email protected]>
Acked-by: Thomas Graf <[email protected]>
Signed-off-by: David S. Miller <[email protected]>