nbd: clear DISCONNECT_REQUESTED flag once disconnection occurs.

When a userspace client requests a NBD device be disconnected, the
DISCONNECT_REQUESTED flag is set. While this flag is set, the driver
will not inform userspace when a connection is closed.

Unfortunately the flag was never cleared, so once a disconnect was
requested the driver would thereafter never tell userspace about a
closed connection. Thus when connections failed due to timeout, no
attempt to reconnect was made and eventually the device would fail.

Fix by clearing the DISCONNECT_REQUESTED flag (and setting the
DISCONNECTED flag) once all connections are closed.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Kevin Vigor <kvigor@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>