Check if gendisk is already up instead of trying to figure out parent
mtd device (which didn't work well).
Reported-by: DENG Qingfang <dqfext@gmail.com>
Reported-by: Koen Vandeputte <koen.vandeputte@citymesh.com>
Fixes: 2809d00007 ("kernel: support FIT partition parser on mtdblock devices")
Signed-off-by: Daniel Golle <daniel@makrotopia.org>