mtd: maps: vmu-flash: Fix the (mtd core) switch to ref counters

mtd: maps: vmu-flash: Fix the (mtd core) switch to ref counters

While switching to ref counters for track mtd devices use, the vmu-flash
driver was forgotten. The reason for reading the ref counter seems
debatable, but let's just fix the build for now.

Fixes: 19bfa9ebebb5 ("mtd: use refcount to prevent corruption")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202312022315.79twVRZw-lkp@intel.com/
Cc: stable@vger.kernel.org
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20231205075936.13831-1-miquel.raynal@bootlin.com