nbd: release exp->blk after all clients are closed

If the socket fd is shutdown, there may be some data which is received before
shutdown. We will read the data and do read/write in nbd_trip(). But the exp's
blk is NULL, and it will cause qemu crashed.

Reported-by: Li Zhijian <lizhijian@cn.fujitsu.com>
Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
Message-Id: <55F929E2.1020501@cn.fujitsu.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>