Revert "Revert "block: nbd: add sanity check for first_minor""

This reverts commit 0daa75bf.

These problems such as:
https://lore.kernel.org/all/CACPK8XfUWoOHr-0RwRoYoskia4fbAbZ7DYf5wWBnv6qUnGq18w@mail.gmail.com/
It was introduced by introduced by commit b1a81163 ("block: nbd: add sanity check for first_minor")
and has been have been fixed by commit e4c4871a

 ("nbd: fix max value for 'first_minor'").

Cc: Joel Stanley <joel@jms.id.au>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Pavel Skripkin <paskripkin@gmail.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Sasha Levin <sashal@kernel.org>
Cc: stable@vger.kernel.org # v5.10+
Signed-off-by: Wen Yang <wenyang.linux@foxmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>