dpif-netlink-rtnl: Use 65000 instead of 65535 as tunnel MTU.

dpif-netlink-rtnl: Use 65000 instead of 65535 as tunnel MTU.

Most of the existing tunnels accept 65535 for MTU and internally reduce it
to the maximum value actually supported.  However, in RTM_SETLINK calls,
at least GRE tunnels reject MTU larger than actually supported.  This
commit changes the MTU used in RTM_NEWLINK calls to use a value that should
be acceptable to all tunnels and yet does not noticeably reduce
performance.

(This code doesn't actually use RTM_SETLINK to change MTU yet, but that's
coming up.)

Suggested-by: Eric Garver <e@erig.me>
Suggested-at: https://mail.openvswitch.org/pipermail/ovs-dev/2018-January/343304.html
Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Eric Garver <e@erig.me>
Tested-by: James Page <james.page@ubuntu.com>