btrfs-progs: add options to sync filesystem after subvol delete

Subvolume deletion does not do a full transaction commit. This can lead
to an unexpected result when the system crashes between deletion and
commit, the subvolume directory will appear again. Add options to request
filesystem sync after each deleted subvolume or after the last one.

If the command with sync option finishes succesfully, the subvolume(s)
deletion status is safely stored on the media.

Userspace approach is more flexible than in-kernel. Related discussions:
http://www.spinics.net/lists/linux-btrfs/msg22088.html
http://www.spinics.net/lists/linux-btrfs/msg27240.html

CC: Alex Lyakas <alex.btrfs@zadarastorage.com>
Signed-off-by: David Sterba <dsterba@suse.cz>