btrfs: Allow rmdir(2) to delete an empty subvolume

Change the behavior of rmdir(2) and allow it to delete an empty
subvolume by using btrfs_delete_subvolume() which is used by
btrfs_ioctl_snap_destroy().

This is a change in behaviour and has been requested by users. Deleting
the subvolume by ioctl requires root permissions while the rmdir way
does works with standard tools and syscalls for all users that can
access the subvolume.

The main usecase is to allow 'rm -rf /path/with/subvols' to simply work.
We were not able to find any nasty usability surprises, the intention is
to do the destructive rm. Without allowing rmdir, this would have to be
followed by the ioctl subvolume deletion, which is more of an annoyance.

Implementation details:

The required lock for @dir and inode of @dentry is already acquired in
vfs layer.

We need some check before deleting a subvolume. Permission check is done
in vfs layer, emptiness check is in btrfs_rmdir() and additional check
(i.e. neither the subvolume is a default subvolume nor send is in progress)
is in btrfs_delete_subvolume().

Note that in btrfs_ioctl_snap_destroy(), d_delete() is called after
btrfs_delete_subvolume(). For rmdir(2), d_delete() is called in vfs
layer later.

Tested-by: Goffredo Baroncelli <kreijack@inwind.it>
Signed-off-by: Tomohiro Misono <misono.tomohiro@jp.fujitsu.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ enhance changelog ]
Signed-off-by: David Sterba <dsterba@suse.com>