sunrpc: Re-purpose trace_svc_process

Currently, trace_svc_process has two call sites:

1. Just after a call to svc_send. svc_send already invokes
   trace_svc_send with the same arguments just before returning

2. Just before a call to svc_drop. svc_drop already invokes
   trace_svc_drop with the same arguments just after it is called

Therefore trace_svc_process does not provide any additional
information not already provided by these other trace points.

However, it would be useful to record the incoming RPC procedure.
So reuse trace_svc_process for this purpose.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>