[NET]: proto_unregister: fix sleeping while atomic

proto_unregister holds a lock while calling kmem_cache_destroy, which
can sleep.

Noticed by Daniele Orlandi <daniele@orlandi.com>.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>