net: fix rtnh_ok()

syzbot reported :

BUG: KMSAN: uninit-value in rtnh_ok include/net/nexthop.h:11 [inline]
BUG: KMSAN: uninit-value in fib_count_nexthops net/ipv4/fib_semantics.c:469 [inline]
BUG: KMSAN: uninit-value in fib_create_info+0x554/0x8d20 net/ipv4/fib_semantics.c:1091

@remaining is an integer, coming from user space.
If it is negative we want rtnh_ok() to return false.

Fixes: 4e902c57

 ("[IPv4]: FIB configuration using struct fib_config")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: David S. Miller <davem@davemloft.net>