neigh: prevent overflowing params in /proc/sys/net/ipv4/neigh/

Without this patch, the fields app_solicit, gc_thresh1, gc_thresh2,
gc_thresh3, proxy_qlen, ucast_solicit, mcast_solicit could have
assumed negative values when setting large numbers.

Signed-off-by: Francesco Fusco <ffusco@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>