SUNRPC: Fix XPT_BUSY flag leakage in svc_handle_xprt()...

If the attempt to reserve a slot fails, we currently leak the XPT_BUSY
flag on the socket. Among other things, this make it impossible to close
the socket.

Fixes: 82011c80

 ("SUNRPC: Move svc_xprt_received() call sites")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>