fib: Use const struct nl_info * in rtmsg_fib

The rtmsg_fib function doesn't modify this argument so mark
it const.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc net/ipv4/fib_lookup.h
Simple merge

diff --cc net/ipv4/fib_semantics.c
Simple merge