net: ipv6: ip6mr: Split ip6mr_forward2() in two

Some of the work of ip6mr_forward2() is specific to IPMR forwarding, and
should not take place on the output path. In order to allow reuse of the
common parts, extract out of the function a helper,
ip6mr_prepare_forward().

Signed-off-by: Petr Machata <petrm@nvidia.com>
Link: https://patch.msgid.link/8932bd5c0fbe3f662b158803b8509604fa7bc113.1750113335.git.petrm@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>