rhashtable: Fix sleeping inside RCU critical section in walk_stop

rhashtable: Fix sleeping inside RCU critical section in walk_stop

The commit 0062d19dc7de879c3c1033f6ea970a6f8e8e8a1f ("rhashtable:
Fix use-after-free in rhashtable_walk_stop") fixed a real bug
but created another one because we may end up sleeping inside an
RCU critical section.

This patch fixes it properly by replacing the mutex with a spin
lock that specifically protects the walker lists.

Reported-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>