ipv6 addrconf: disallow rtr_solicits < -1

This disallows setting /proc/sys/net/ipv6/conf/*/router_solicitations
to values below -1.

-1 continues to mean an unlimited number of retransmits.

Note: this depends on 'ipv6 addrconf: remove addrconf_sysctl_hop_limit()'

Signed-off-by: Maciej Żenczykowski <maze@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>