btrfs: Use sizeof directly instead of a constant variable

The kernel would like to have all stack VLA usage removed[1].
Unfortunately using an integer constant variable as the size of an
array is still considered a VLA. Instead let's use directly sizeof(var)
which removes the VLA usage. Use the occasion to remove csum_size
altogether and use sizeof() also for the size passed to memcmp

[1]: https://lkml.org/lkml/2018/3/7/621

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -403,8 +403,7 @@ static int btrfs_check_super_csum(struct btrfs_fs_info *fs_info,
 
 	if (csum_type == BTRFS_CSUM_TYPE_CRC32) {
 		u32 crc = ~(u32)0;
-		const int csum_size = sizeof(crc);
-		char result[csum_size];
+		char result[sizeof(crc)];
 
 		/*
 		 * The super_block structure does not span the whole
@@ -415,7 +414,7 @@ static int btrfs_check_super_csum(struct btrfs_fs_info *fs_info,
 				crc, BTRFS_SUPER_INFO_SIZE - BTRFS_CSUM_SIZE);
 		btrfs_csum_final(crc, result);
 
-		if (memcmp(raw_disk_sb, result, csum_size))
+		if (memcmp(raw_disk_sb, result, sizeof(result)))
 			ret = 1;
 	}