net: Flush GRO list when input packet queue is empty

Commit 11ef7a89 ("net: Performance fix for process_backlog")
modifies process_backlog() to no longer check for new packets once
either quota is met or input_pkt_queue is empty.
As a result, the napi_gro_flush() added by Commit I05bde8bb00c4ec
("net: adding API to get the current NAPI context") was no longer
taking effect. This was causing TCP timeouts to occur often and
TCP was getting stuck in slow start since the GRO list was not
getting flushed.

Fix this by calling napi_gro_flush when the input_pkt_queue is
determined to be empty.

CRs-fixed: 871247
Change-Id: I9f2ede25d1cbe89e970d65a2843958385803563c
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>