mtd_blkdevs: don't scan partitions for plain mtdblock

mtdblock / mtdblock_ro set part_bits to 0 and thus nevever scanned
partitions.  Restore that behavior by setting the GENHD_FL_NO_PART flag.

Fixes: 1ebe2e5f ("block: remove GENHD_FL_EXT_DEVT")
Reported-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20211206070409.2836165-1-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>