__memblock_remove()'s open coded region manipulation can be trivially
replaced with memblock_islate_range().  This increases code sharing
and eases improving region tracking.

This pulls memblock_isolate_range() out of HAVE_MEMBLOCK_NODE_MAP.
Make it use memblock_get_region_node() instead of assuming rgn->nid is
available.

-v2: Fixed build failure on !HAVE_MEMBLOCK_NODE_MAP caused by direct
     rgn->nid access.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Yinghai Lu <yinghai@kernel.org>