If protocols registered exceeded PROTO_INUSE_NR, prot will be
added to proto_list, but no available bit left for prot in
proto_inuse_idx.

Changes since v2:
* Propagate the error code properly
Signed-off-by: Nzhanglin <zhang.lin16@zte.com.cn>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>