rhashtable: Call library function alloc_bucket_locks

To allocate the array of bucket locks for the hash table we now
call library function alloc_bucket_spinlocks. This function is
based on the old alloc_bucket_locks in rhashtable and should
produce the same effect.

Signed-off-by: Tom Herbert <tom@quantonium.net>
Signed-off-by: David S. Miller <davem@davemloft.net>