udp: document udp_rcv_segment special case for looped packets

udp: document udp_rcv_segment special case for looped packets

Commit 904b0b17ea15c ("udp: segment looped gso packets correctly")
fixes an issue with rare udp gso multicast packets looped onto the
receive path.

The stable backport makes the narrowest change to target only these
packets, when needed. As opposed to, say, expanding __udp_gso_segment,
which is harder to reason to be free from unintended side-effects.

But the resulting code is hardly self-describing.
Document its purpose and rationale.

Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>