net: ipv6: Add sysctl for minimum prefix len acceptable in RIOs.

This commit adds a new sysctl accept_ra_rt_info_min_plen that
defines the minimum acceptable prefix length of Route Information
Options. The new sysctl is intended to be used together with
accept_ra_rt_info_max_plen to configure a range of acceptable
prefix lengths. It is useful to prevent misconfigurations from
unintentionally blackholing too much of the IPv6 address space
(e.g., home routers announcing RIOs for fc00::/7, which is
incorrect).

Signed-off-by: Joel Scherpelz <jscherpelz@google.com>
Acked-by: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/ipv6.h

@@ -184,6 +184,7 @@ enum {
 	DEVCONF_ENHANCED_DAD,
 	DEVCONF_ADDR_GEN_MODE,
 	DEVCONF_DISABLE_POLICY,
+	DEVCONF_ACCEPT_RA_RT_INFO_MIN_PLEN,
 	DEVCONF_MAX
 };