nfsd4: centralize more calls to svc_xprt_received

Follow up on b48fa6b9 by moving all the
svc_xprt_received() calls for the main xprt to one place.  The clearing
of XPT_BUSY here is critical to the correctness of the server, so I'd
prefer it to be obvious where we do it.

The only substantive result is moving svc_xprt_received() after
svc_receive_deferred().  Other than a (likely insignificant) delay
waking up the next thread, that should be harmless.

Also reshuffle the exit code a little to skip a few other steps that we
don't care about the in the svc_delete_xprt() case.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>