nbd: don't update block size after device is started

nbd: don't update block size after device is started

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: [email protected]
Fixes: c8a83a6b54d0 ("nbd: Use set_blocksize() to set device blocksize")
Reported-by: lining <[email protected]>
Signed-off-by: Ming Lei <[email protected]>
Cc: Josef Bacik <[email protected]>
Cc: Jan Kara <[email protected]>
Tested-by: lining <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>