Staging: zram: Fix access of NULL pointer

This patch fixes the bug in reset_store caused by accessing NULL pointer.

The bdev gets its value from bdget_disk() which could fail when memory
pressure is severe and hence can return NULL because allocation of
inode in bdget could fail.

Hence, this patch introduces a check for bdev to prevent reference to a
NULL pointer in the later part of the code. It also removes unnecessary
check of bdev for fsync_bdev().

Cc: stable <stable@vger.kernel.org>
Acked-by: Jerome Marchand <jmarchan@redhat.com>
Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/zram/zram_drv.c

@@ -648,6 +648,9 @@ static ssize_t reset_store(struct device *dev,
 	zram = dev_to_zram(dev);
 	bdev = bdget_disk(zram->disk, 0);
 
+	if (!bdev)
+		return -ENOMEM;
+
 	/* Do not reset an active device! */
 	if (bdev->bd_holders)
 		return -EBUSY;
@@ -660,7 +663,6 @@ static ssize_t reset_store(struct device *dev,
 		return -EINVAL;
 
 	/* Make sure all pending I/O is finished */
-	if (bdev)
 	fsync_bdev(bdev);
 
 	zram_reset_device(zram, true);