Use lockdep_assert_held to ensure mtd_table_mutex is held instead of
mutex_trylock games.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210823073359.705281-3-hch@lst.de