Btrfs: remove ourselves from the cluster list under lock

commit b8d0c69b9469ffd33df30fee3e990f2d4aa68a09 upstream.

A user was reporting weird warnings from btrfs_put_delayed_ref() and I noticed
that we were doing this list_del_init() on our head ref outside of
delayed_refs->lock.  This is a problem if we have people still on the list, we
could end up modifying old pointers and such.  Fix this by removing us from the
list before we do our run_delayed_ref on our head ref.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>