mtd: don't lock when recursively deleting partitions

When recursively deleting partitions, don't acquire the masters
partition lock twice. Otherwise the process ends up in a deadlocked
state.

Fixes: 46b5889c ("mtd: implement proper partition handling")
Signed-off-by: David Bauer <mail@david-bauer.net>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210217195320.893253-1-mail@david-bauer.net