Revert "GTP: add support for flow based tunneling API"

Revert "GTP: add support for flow based tunneling API"

This reverts commit 9ab7e76aefc97a9aa664accb59d6e8dc5e52514a.

This patch was committed without maintainer approval and despite a number
of unaddressed concerns from review.  There are several issues that
impede the acceptance of this patch and that make a reversion of this
particular instance of these changes the best way forward:

i)  the patch contains several logically separate changes that would be
better served as smaller patches (for review purposes)
ii) functionality like the handling of end markers has been introduced
without further explanation
iii) symmetry between the handling of GTPv0 and GTPv1 has been
unnecessarily broken
iv) the patchset produces 'broken' packets when extension headers are
included
v) there are no available userspace tools to allow for testing this
functionality
vi) there is an unaddressed Coverity report against the patch concering
memory leakage
vii) most importantly, the patch contains a large amount of superfluous
churn that impedes other ongoing work with this driver

This patch will be reworked into a series that aligns with other
ongoing work and facilitates review.

Signed-off-by: Jonas Bonn <jonas@norrbonn.se>
Acked-by: Harald Welte <laforge@gnumonks.org>
Acked-by: Pravin B Shelar <pshelar@ovn.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>