powerpc/mm: fix "section_base" set but not used

The commit 24b6d416 ("mm: pass the vmem_altmap to vmemmap_free")
removed a line in vmemmap_free(),

altmap = to_vmem_altmap((unsigned long) section_base);

but left a variable no longer used.

arch/powerpc/mm/init_64.c: In function 'vmemmap_free':
arch/powerpc/mm/init_64.c:277:16: error: variable 'section_base' set but
not used [-Werror=unused-but-set-variable]
Signed-off-by: Qian Cai <cai@lca.pw>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/mm/init_64.c

@@ -274,7 +274,6 @@ void __ref vmemmap_free(unsigned long start, unsigned long end,
 
 	for (; start < end; start += page_size) {
 		unsigned long nr_pages, addr;
-		struct page *section_base;
 		struct page *page;
 
 		/*
@@ -290,7 +289,6 @@ void __ref vmemmap_free(unsigned long start, unsigned long end,
 			continue;
 
 		page = pfn_to_page(addr >> PAGE_SHIFT);
-		section_base = pfn_to_page(vmemmap_section_start(start));
 		nr_pages = 1 << page_order;
 		base_pfn = PHYS_PFN(addr);