uprobes: _unregister() should always do register_for_each_vma(false)

uprobe_unregister() removes the breakpoints only if the last consumer
goes away. To support the filtering it should do this every time, we
want to remove the breakpoints which nobody else want to keep.

Note: given that filter_chain() is not actually implemented, this patch
itself doesn't change the behaviour yet, register_for_each_vma(false)
is a heavy "nop" unless there are no more consumers.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>