tipc: add missing attribute validation for MTU property

[ Upstream commit 213320a67962ff6e7b83b704d55cbebc341426db ]

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>