memblock: __next_mem_pfn_range_in_zone: remove unneeded local variable nid

The nid is only used to act as output parameter of __next_mem_range.
Since NULL can be passed to __next_mem_range as out_nid, we can thus
remove nid by passing NULL here.

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
[rppt: updated the commit message]
Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>