mtd: Add check for devm_kcalloc()

[ Upstream commit 2aee30bb ]

Add a check for devm_kcalloc() to ensure successful allocation.

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>
[ Adjust context ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>