Mounted NBD device can be resized, one use case is rbd-nbd.

Fix the issue by setting up default block size, then not touch it
in nbd_size_update() any more. This kind of usage is aligned with loop
which has same use case too.

Cc: stable@vger.kernel.org
Fixes: c8a83a6b ("nbd: Use set_blocksize() to set device blocksize")
Reported-by: Nlining <lining2020x@163.com>
Signed-off-by: NMing Lei <ming.lei@redhat.com>
Cc: Josef Bacik <josef@toxicpanda.com>
Cc: Jan Kara <jack@suse.cz>
Tested-by: Nlining <lining2020x@163.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>