net: adding API to get the current NAPI context

Adding a new API - get_current_napi_context(), which returns the
current napi_struct on the current running CPU.
This change allows network drivers to receive the current
running napi_struct, to use when calling APIs that require it.

Adding a new field to struct softnet_data: current_napi.
The field current_napi will hold the current running napi_sturct.

Adding a call to napi_gro_flush() from process_backlog() when there
are no more packets to process, since process_backlog() doesn't
call napi_complete() API.

CRs-fixed: 784626
Change-Id: I05bde8bb00c4ec6440fc1db29e741746b759d1b7
Signed-off-by: Sivan Reinstein <sivanr@codeaurora.org>