btrfs: move fs_info forward declarations to the top of ctree.h

In order to make it more straightforward to move the fs_info struct and
it's related structures, move the struct declarations to the top of
ctree.h.  This will make it easier to clean up after the fact.
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.h

@@ -49,6 +49,11 @@ struct btrfs_ordered_sum;
 struct btrfs_ref;
 struct btrfs_bio;
 struct btrfs_ioctl_encoded_io_args;
+struct btrfs_device;
+struct btrfs_fs_devices;
+struct btrfs_balance_control;
+struct btrfs_delayed_root;
+struct reloc_control;
 
 #define BTRFS_MAGIC 0x4D5F53665248425FULL /* ascii _BHRfS_M, no null */
 
@@ -528,13 +533,6 @@ struct btrfs_discard_ctl {
 	atomic64_t discard_bytes_saved;
 };
 
-/* fs_info */
-struct reloc_control;
-struct btrfs_device;
-struct btrfs_fs_devices;
-struct btrfs_balance_control;
-struct btrfs_delayed_root;
-
 enum {
 	BTRFS_FS_CLOSING_START,
 	BTRFS_FS_CLOSING_DONE,