net: rmnet_data: Remove the NAPI context for rmnet_data devices

Commit 28207b51 ("net: rmnet_data: Add NAPI context for rmnet_data
devices") added a NAPI struct per rmnet_data device. This was to
ensure that the NAPI struct is always available even if there was a
hotplug. However, this seems to be leading to some races where the
NAPI struct is accessed concurrently across cores.

The race here is between napi_gro_receive on one core with
napi_complete running on the other accessing the same NAPI struct.
If napi_gro_receive runs slightly earlier, napi_complete would see
that the napi->gro_list would be non NULL in __napi_complete even
though it had cleared earlier and would lead to a BUG.
If napi_complete runs slightly earlier, napi_gro_receive would
dereference a NULL pointer even though it had assigned an skb to
napi->gro_list.

Fix this by using the per cpu backlog struct as the NAPI struct for
queuing packets to GRO engine. Access across cores would not be a
problem in case of hotplug as they would use core specific structures.

CRs-Fixed: 966095
Change-Id: I831df5b93cc6ee77355f2e98af89efcffe825bd8
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>