btrfs: support recursive subvolume deletion

Pull the #defines and struct definitions for btrfs into a separate
.h file to not clutter bdev.c

Implement btrfs recursive delete support

A non-root user isn't allow to do the ioctls needed for searching (as you can
verify with 'btrfs subvolume list').  So for an unprivileged user, if the
rootfs has subvolumes under it, deletion will fail.  Otherwise, it will
succeed.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>