mm: unexport free_reserved_area

This function is only used by built-in code, which makes perfect sense
given the purpose of it.

Link: http://lkml.kernel.org/r/20190213174621.29297-2-hch@lst.de
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -7341,7 +7341,6 @@ unsigned long free_reserved_area(void *start, void *end, int poison, const char
 
 	return pages;
 }
-EXPORT_SYMBOL(free_reserved_area);
 
 #ifdef	CONFIG_HIGHMEM
 void free_highmem_page(struct page *page)