tipc: add missing attribute validation for MTU property

Add missing attribute validation for TIPC_NLA_PROP_MTU
to the netlink policy.

Fixes: 901271e0

 ("tipc: implement configuration of UDP media MTU")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>