tipc: check the bearer min mtu properly when setting it by netlink

Checking the bearer min mtu with tipc_udp_mtu_bad() only works for
IPv4 UDP bearer, and IPv6 UDP bearer has a different value for the
min mtu. This patch checks with encap_hlen + TIPC_MIN_BEARER_MTU
for min mtu, which works for both IPv4 and IPv6 UDP bearer.

Note that tipc_udp_mtu_bad() is still used to check media min mtu
in __tipc_nl_media_set(), as m->mtu currently is only used by the
IPv4 UDP bearer as its default mtu value.

Fixes: 682cd3cf ("tipc: confgiure and apply UDP bearer MTU on running links")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Jon Maloy <jmaloy@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>