Changes

ggsn: Avoid forwarding IPv6 solicited-node multicast addr to tun device

There's no need to forward those. Morevoer, they cannot be forwarded
when using gtpu kernel mode since it doesn't support reinjecting packets
we got from it, so avoid erroring out when trying to inject them later
on.

Related: OS#6600
Related: OS#6382
Change-Id: Iac8f14083620d86f3872aa951930fbe8f680ff24

ggsn: Rename confusing functions

It's really confusing having a function operating on a rx path called
"encaps" since it's not really encapsulating anything.
This confuses the reader by thinking this is the function encapsulating
packets into GTP-U.

Change-Id: I7ff54f5e57ccc2c436becd1be1b0f728cbcdb12c

tun.h: Remove non-existent tun_decaps()

Change-Id: If9cc5d2d54b87e38feefa55cb9410715fa9bd11f

Rename tun_encaps -> tun_inject_pkt

The terminology of "encapsulating" used in tun_encaps is confusing,
since in this case we are not really encapsulating anything, but
actually delivering ("injecting") the packet to the system network
stack.
Using the "encapsulating" term is also confusing because readers may
think it may be doing the GTP-U encapsulation, which is not.

Change-Id: Ibb17c792b81668571e69d79918f3adf0e9e0f6c8