btrfs: Move may_destroy_subvol() from ioctl.c to inode.c

This is a preparation work to refactor btrfs_ioctl_snap_destroy()
and to allow rmdir(2) to delete an empty subvolume.

Signed-off-by: Tomohiro Misono <misono.tomohiro@jp.fujitsu.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ minor update of the function comment ]
Signed-off-by: David Sterba <dsterba@suse.com>