nfsd: ensure sockets are closed on error

One the changes in commit d7979ae4 "svc: Move close processing to a
single place" is:

  err_delete:
-       svc_delete_socket(svsk);
+       set_bit(SK_CLOSE, &svsk->sk_flags);
        return -EAGAIN;

This is insufficient. The recvfrom methods must always call
svc_xprt_received on completion so that the socket gets re-queued if
there is any more work to do.  This particular path did not make that
call because it actually destroyed the svsk, making requeue pointless.
When the svc_delete_socket was change to just set a bit, we should have
added a call to svc_xprt_received,

This is the problem that b0401d72 attempted to fix, incorrectly.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>