mtd: use refcount to prevent corruption

When underlying device is removed mtd core will crash
in case user space is holding open handle.
Need to use proper refcounting so device is release
only when has no users.

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Alexander Usyskin <alexander.usyskin@intel.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20230620131905.648089-2-alexander.usyskin@intel.com