btrfs: remove the zoned/zone_size union in struct btrfs_fs_info

Reading a value from a different member of a union is not just a great
way to obfuscate code, but also creates an aliasing violation.  Switch
btrfs_is_zoned to look at ->zone_size and remove the union.

Note: union was to simplify the detection of zoned filesystem but now
this is wrapped behind btrfs_is_zoned so we can drop the union.
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Naohiro Aota <naohiro.aota@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
[ add note ]
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.h

@@ -1045,10 +1045,7 @@ struct btrfs_fs_info {
 	 * Zone size > 0 when in ZONED mode, otherwise it's used for a check
 	 * if the mode is enabled
 	 */
-	union {
-		u64 zone_size;
-		u64 zoned;
-	};
+	u64 zone_size;
 
 	struct mutex zoned_meta_io_lock;
 	spinlock_t treelog_bg_lock;
@@ -4010,7 +4007,7 @@ static inline int btrfs_is_testing(struct btrfs_fs_info *fs_info)
 
 static inline bool btrfs_is_zoned(const struct btrfs_fs_info *fs_info)
 {
-	return fs_info->zoned != 0;
+	return fs_info->zone_size > 0;
 }
 
 static inline bool btrfs_is_data_reloc_root(const struct btrfs_root *root)