nfs41: Refactor svc_process()

net/sunrpc/svc.c:svc_process() is used by the NFSv4 callback service
to process RPC requests arriving over connections initiated by the
server.  NFSv4.1 supports callbacks over the backchannel on connections
initiated by the client.  This patch refactors svc_process() so that
common code can also be used by the backchannel.

Signed-off-by: Ricardo Labiaga <ricardo.labiaga@netapp.com>
Signed-off-by: Benny Halevy <bhalevy@panasas.com>