btrfs: directly wake up cleaner kthread in the BTRFS_IOC_SYNC ioctl

The BTRFS_IOC_SYNC ioctl wants to wake up the cleaner kthread so that it
does any pending work (subvolume deletion, delayed iputs, etc), however
it is waking up the transaction kthread, which in turn wakes up the
cleaner. Since we don't have any transaction to commit, as any ongoing
transaction was already committed when it called btrfs_sync_fs() and
the goal is just to wake up the cleaner thread, directly wake up the
cleaner instead of the transaction kthread.

Reviewed-by: Boris Burkov <boris@bur.io>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>