[VLAN]: Move vlan_group allocation to seperate function

Move group allocation to a seperate function to clean up the code a bit
and allocate groups before registering the device. Device registration
is globally visible and causes netlink events, so we shouldn't fail
afterwards.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>