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When the last thread of nfsd exits, it shuts down all related sockets.
It currently uses svc_close_socket to do this, but that only is
immediately effective if the socket is not SK_BUSY.

If the socket is busy - i.e. if a request has arrived that has not yet
been processes - svc_close_socket is not effective and the shutdown
process spins.

So create a new svc_force_close_socket which removes the SK_BUSY flag
is set and then calls svc_close_socket.

Also change some open-codes loops in svc_destroy to use
list_for_each_entry_safe.
Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>