btrfs: support recursive subvolume deletion (v2)

btrfs: support recursive subvolume deletion (v2)

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.

Changelog: Aug 1:
  . Fix wrong objid passing when determining directory paths
  . In do_remove_btrfs_children, avoid dereferencing NULL dirid
  . Fix memleak in error case.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>