NFSD: Hoist status code encoding into XDR encoder functions

The original intent was presumably to reduce code duplication. The
trade-off was:

- No support for an NFSD proc function returning a non-success
  RPC accept_stat value.
- No support for void NFS replies to non-NULL procedures.
- Everyone pays for the deduplication with a few extra conditional
  branches in a hot path.

In addition, nfsd_dispatch() leaves *statp uninitialized in the
success path, unlike svc_generic_dispatch().

Address all of these problems by moving the logic for encoding
the NFS status code into the NFS XDR encoders themselves. Then
update the NFS .pc_func methods to return an RPC accept_stat
value.

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