ipv4: fix refcount leak in fib_check_nh()

fib_lookup() forces FIB_LOOKUP_NOREF flag, while fib_table_lookup()
does not.

This patch solves the typical message at reboot time or device
dismantle :

unregister_netdevice: waiting for eth0 to become free. Usage count = 4

Fixes: 3bfd8472 ("net: Use passed in table for nexthop lookups")
Signed-off-by: NEric Dumazet <edumazet@google.com>
Cc: David Ahern <dsa@cumulusnetworks.com>
Acked-by: NDavid Ahern <dsa@cumulusnetworks.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/fib_semantics.c

@@ -708,7 +708,8 @@ static int fib_check_nh(struct fib_config *cfg, struct fib_info *fi,
 
 			if (tbl)
 				err = fib_table_lookup(tbl, &fl4, &res,
-						   FIB_LOOKUP_IGNORE_LINKSTATE);
+						       FIB_LOOKUP_IGNORE_LINKSTATE |
+						       FIB_LOOKUP_NOREF);
 			else
 				err = fib_lookup(net, &fl4, &res,
 						 FIB_LOOKUP_IGNORE_LINKSTATE);