net: Add the get current NAPI context API

Commit 69235aa80090 ("net: Remove the get current NAPI context API")
removed the definition of get_current_napi_context() as rmnet_data
was no longer using it. However, the rmnet_data change to use its
NAPI in multiple contexts was prone to race in hotplug scenarios.

Add back get_current_napi_context() and current_napi to the
softnet_data struct.

Change-Id: I494112f908c2222ff29cde049620d338c44d18c2
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>