vxlan: restore dev->mtu setting based on lower device

Stefano Brivio says:
    Commit a985343b ("vxlan: refactor verification and
    application of configuration") introduced a change in the
    behaviour of initial MTU setting: earlier, the MTU for a link
    created on top of a given lower device, without an initial MTU
    specification, was set to the MTU of the lower device minus
    headroom as a result of this path in vxlan_dev_configure():

	if (!conf->mtu)
		dev->mtu = lowerdev->mtu -
			   (use_ipv6 ? VXLAN6_HEADROOM : VXLAN_HEADROOM);

    which is now gone. Now, the initial MTU, in absence of a
    configured value, is simply set by ether_setup() to ETH_DATA_LEN
    (1500 bytes).

    This breaks userspace expectations in case the MTU of
    the lower device is higher than 1500 bytes minus headroom.

This patch restores the previous behaviour on newlink operation. Since
max_mtu can be negative and we update dev->mtu directly, also check it
for valid minimum.

Reported-by: Junhan Yan <juyan@redhat.com>
Fixes: a985343b ("vxlan: refactor verification and application of configuration")
Signed-off-by: Alexey Kodanev <alexey.kodanev@oracle.com>
Acked-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>