mm/nobootmem.c: return 0 when start_pfn equals end_pfn

When start_pfn equals end_pfn, __free_pages_memory() has no effect and
__free_memory_core() will finally return (end_pfn - start_pfn) = 0.

This patch returns 0 directly when start_pfn equals end_pfn.

Link: http://lkml.kernel.org/r/20170502131115.6650-1-richard.weiyang@gmail.com
Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: RichardWeiYang

mm/nobootmem.c

@@ -118,7 +118,7 @@ static unsigned long __init __free_memory_core(phys_addr_t start,
 	unsigned long end_pfn = min_t(unsigned long,
 				      PFN_DOWN(end), max_low_pfn);
 
-	if (start_pfn > end_pfn)
+	if (start_pfn >= end_pfn)
 		return 0;
 
 	__free_pages_memory(start_pfn, end_pfn);