geneve: fix build with modular IPV6

Commit c40e89fd ("geneve: configure MTU based on a lower device") added
an IS_ENABLED(CONFIG_IPV6) to geneve, leading to the following link error
with CONFIG_GENEVE=y and CONFIG_IPV6=m:

drivers/net/geneve.o: In function `geneve_link_config':
geneve.c:(.text+0x14c): undefined reference to `rt6_lookup'

Fix this by adding a Kconfig dependency and forcing GENEVE to be a module
when IPV6 is a module.

Fixes: c40e89fd ("geneve: configure MTU based on a lower device")
Signed-off-by: Tobias Regnery <tobias.regnery@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>