SUNRPC move svc_drop to caller of svc_process_common

The NFSv4.1 shared back channel does not need to call svc_drop because the
callback service never outlives the single connection it services, and it
reuses it's buffers and keeps the trasport.

Signed-off-by: Andy Adamson <andros@netapp.com>
Acked-by: Bruce Fields <bfields@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>