btrfs: change btrfs_fs_devices::seeding to bool

struct btrfs_fs_devices::seeding currently is declared as an integer
variable but only used as a boolean.

Change the variable definition to bool and update to code touching it to
set 'true' and 'false'.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/volumes.h

@@ -243,7 +243,7 @@ struct btrfs_fs_devices {
 	struct list_head alloc_list;
 
 	struct btrfs_fs_devices *seed;
-	int seeding;
+	bool seeding;
 
 	int opened;