mm: fix section mismatch warning in sparse.c

Fix following warning:
WARNING: mm/built-in.o(.text+0x22069): Section mismatch in reference from the function sparse_early_usemap_alloc() to the function .init.text:__alloc_bootmem_node()

static sparse_early_usemap_alloc() were used only by sparse_init()
and with sparse_init() annotated _init it is safe to
annotate sparse_early_usemap_alloc with __init too.
Signed-off-by: NSam Ravnborg <sam@ravnborg.org>
Cc: Andy Whitcroft <apw@shadowen.org>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Christoph Lameter <clameter@sgi.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/sparse.c

@@ -237,7 +237,7 @@ static unsigned long *__kmalloc_section_usemap(void)
 }
 #endif /* CONFIG_MEMORY_HOTPLUG */
 
-static unsigned long *sparse_early_usemap_alloc(unsigned long pnum)
+static unsigned long *__init sparse_early_usemap_alloc(unsigned long pnum)
 {
 	unsigned long *usemap;
 	struct mem_section *ms = __nr_to_section(pnum);