null_blk: suppress invalid partition info

null_blk is partitionable, but it doesn't store any of the info. When
it is loaded, you would normally see:

[1226739.343608]  nullb0: unknown partition table
[1226739.343746]  nullb1: unknown partition table

which can confuse some people. Add the appropriate gendisk flag
to suppress this info.

Signed-off-by: Jens Axboe <axboe@fb.com>