geneve: check MTU for a minimum in geneve_change_mtu()

geneve_change_mtu() will be used not only as ndo_change_mtu() callback,
but also to verify a user specified MTU on a new link creation in the
next patch.

Signed-off-by: Alexey Kodanev <alexey.kodanev@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>