UPSTREAM: udp: properly flush normal packet at GRO time

If an UDP packet enters the GRO engine but is not eligible
for aggregation and is not targeting an UDP tunnel,
udp_gro_receive() will not set the flush bit, and packet
could delayed till the next napi flush.

Fix the issue ensuring non GROed packets traverse
skb_gro_flush_final().

Reported-and-tested-by: Matthias Treydte <mt@waldheinz.de>
Fixes: 18f25dc39990 ("udp: skip L4 aggregation for UDP tunnel packets")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit b43c8909be52f2baca8884f967b418a88424494a)
Bug: 187129171
Signed-off-by: Connor O'Brien <connoro@google.com>
Change-Id: Ic240b52b00c04621d0f523c1540e6d4b587ad1cd