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

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

This reverts commit fb2900af4cf7319523ff8f71c35a3b5ef499ad6a which is
commit b1a811633f7321cf1ae2bb76a66805b7720e44c9 upstream.

The backport of this is reported to be causing some problems, so revert
this for now until they are worked out.

Link: https://lore.kernel.org/r/CACPK8XfUWoOHr-0RwRoYoskia4fbAbZ7DYf5wWBnv6qUnGq18w@mail.gmail.com
Reported-by: 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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>