It is not related to init_memory_mapping(),  and init_memory_mapping() is
getting more bigger.

So make it as seperated function and call it from reserve_brk() and that is
point when _brk_end is concluded.
Signed-off-by: NYinghai Lu <yinghai@kernel.org>
LKML-Reference: <4D1933E0.7090305@kernel.org>
Signed-off-by: NH. Peter Anvin <hpa@linux.intel.com>