btrfs-progs: fix the wrong size from device_get_partition_size_sysfs()

[BUG]
When an unprivileged user, who can not access the block device, run
"btrfs dev usage", it's very common to result the following incorrect
output:

  $ btrfs dev usage /mnt/btrfs/
  WARNING: cannot read detailed chunk info, per-device usage will not be shown, run as root
  /dev/mapper/test-scratch1, ID: 1
     Device size:            20.00MiB <<<
     Device slack:           16.00EiB <<<
     Unallocated:                 N/A

Note if the unprivileged user has read access to the raw block file, it
will work as expected:

  $ btrfs dev usage /mnt/btrfs/
  WARNING: cannot read detailed chunk info, per-device usage will not be shown, run as root
  /dev/mapper/test-scratch1, ID: 1
     Device size:            10.00GiB
     Device slack:              0.00B
     Unallocated:                 N/A

[CAUSE]
When device_get_partition_size() is called, firstly the function checks
if we can do a read-only open() on the block device.

However under most distros, block devices are only accessible by root
and "disk" group.

If the unprivileged user is not in "disk" group, the open() will fail
and we have to fallback to device_get_partition_size_sysfs() as the
fallback.

The function device_get_partition_size_sysfs() will use
"/sys/block/<device>/size" as the size of the disk.

But according to the kernel source code, the "size" attribute is
implemented by returning bdev_nr_sectors(), and that result is always in
sector unit (512 bytes).

So if device_get_partition_size_sysfs() returns the value directly, it's
512 times smaller than the original size, causing errors.

[FIX]
Just do the proper left shift to return size in bytes.

Issue: #979
Signed-off-by: Qu Wenruo <[email protected]>

Author: raczzoli

common/device-utils.c

@@ -369,7 +369,9 @@ static u64 device_get_partition_size_sysfs(const char *dev)
 		return 0;
 	}
 	close(sysfd);
-	return size;
+
+	/* <device>/size value is in sector (512B) unit. */
+	return size << SECTOR_SHIFT;
 }
 
 u64 device_get_partition_size(const char *dev)