geneve: move all configuration under struct geneve_config

This patch adds a new structure geneve_config and moves the per-device
configuration attributes to it, like we already have in VXLAN with
struct vxlan_config. This ends up being pretty invasive since those
attributes are used everywhere.

This allows us to clean up the argument lists for geneve_configure (4
arguments instead of 8) and geneve_nl2info (5 instead of 9).

This also reduces the copy-paste of code setting those attributes
between geneve_configure and geneve_changelink to a single memcpy,
which would have avoided the bug fixed in commit
56c09de3 ("geneve: allow changing DF behavior after creation").
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: David S. Miller <davem@davemloft.net>