vxlan: allow for dynamic source ip selection (FS#3426)

By setting 'auto', the zero address or the empty string as source
address (option ipaddr, option ip6addr), vxlan will choose one
dynamically. This helps in setups where a wan ip or prefix changes.

This corresponse to setting up an vxlan tunnel with:

proto vxlan6:
    # ip link add vx0 type vxlan id ID local :: ...
proto vxlan:
    # ip link add vx0 type vxlan id ID local 0.0.0.0 ...

While it is possible to not specify a source ip at all, the kernel will
default to setting up a ipv4 tunnel. The kernel will take any hint from
source and peer ips to figure out, what tunnel type to use. To make sure
we setup an ipv6 tunnel for proto vxlan6, this workaround is needed.

This will not change the behaviour of currently working configurations.
However this will allow former broken configurations, namely those not
specifying both a source address and tunnel interface, to setup a
tunnel interface. Previously those configurations weren't reporting an
error and were stueck in a setup loop like in Bug FS#3426.

This change lifts the currently very strict behaviour and should fix the
following bug:

Fixes: FS#3426
Ref: https://bugs.openwrt.org/index.php?do=details&task_id=3426

Signed-off-by: Johannes Kimmel <fff@bareminimum.eu>

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