netlink: Move mutex_is_held under PROVE_LOCKING

The rhashtable function mutex_is_held is only used when PROVE_LOCKING
is enabled.  This patch modifies netlink so that we can rhashtable.h
itself can later make mutex_is_held optional depending on PROVE_LOCKING.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>