mtd: char: Get rid of Big MTD Lock

Get rid of central chrdev MTD lock, which prevents simultaneous operations
on completely independent physical MTD chips. Replace it with newly
introduced per-master mutex.

Signed-off-by: Alexander Sverdlin <alexander.sverdlin@nokia.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210217211845.43364-2-alexander.sverdlin@nokia.com