net: Make sure BHs are disabled in sock_prot_inuse_add()

There is still a call to sock_prot_inuse_add() in af_netlink
while in a preemptable section. Add explicit BH disable around
this call.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>