net: gro: fix outer network offset

In the Linux kernel, the following vulnerability has been resolved:

net: gro: fix outer network offset

The udp GRO complete stage assumes that all the packets inserted the RX

have the `encapsulation` flag zeroed. Such assumption is not true, as a

few H/W NICs can set such flag when H/W offloading the checksum for

an UDP encapsulated traffic, the tun driver can inject GSO packets with

UDP encapsulation and the problematic layout can also be created via

a veth based setup.

Due to the above, in the problematic scenarios, udp4_gro_complete() uses

the wrong network offset (inner instead of outer) to compute the outer

UDP header pseudo checksum, leading to csum validation errors later on

in packet processing.

Address the issue always clearing the encapsulation flag at GRO completion

time. Such flag will be set again as needed for encapsulated packets by

udp_gro_complete().