The ipip tunnel introduced in commit dd9ee344 ("vti4: Fix a ipip
packet processing bug in 'IPCOMP' virtual tunnel") largely duplicated
the existing vti_input and vti_recv functions.  Refactored to
deduplicate the common code.
Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>