x86-64: Move out cleanup higmap [_brk_end, _end) out of init_memory_mapping()

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: Yinghai Lu <yinghai@kernel.org>
LKML-Reference: <4D1933E0.7090305@kernel.org>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>