mtd: Replace kcalloc() with devm_kcalloc()

commit 1b61a598 upstream.

Replace kcalloc() with devm_kcalloc() to prevent memory leaks in case of
errors.

Fixes: 78c08247 ("mtd: Support kmsg dumper based on pstore/blk")
Cc: stable@vger.kernel.org # v5.10+
Signed-off-by: Jiasheng Jiang <jiashengjiangcool@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>