x86/mm: Declare 'start' variable where it is used

It is not required to initialize the local variable start in
memory_map_top_down(), as the variable will be initialized in any path
before it is used.

make clang-analyzer on x86_64 tinyconfig reports:

  arch/x86/mm/init.c:612:15: warning: Although the value stored to 'start' \
  is used in the enclosing expression, the value is never actually read \
  from 'start' [clang-analyzer-deadcode.DeadStores]

Move the variable declaration into the loop, where it is used.

No code changed:

  # arch/x86/mm/init.o:

   text    data     bss     dec     hex filename
   7105    1424   26768   35297    89e1 init.o.before
   7105    1424   26768   35297    89e1 init.o.after

md5:
   a8d76c1bb5fce9cae251780a7ee7730f  init.o.before.asm
   a8d76c1bb5fce9cae251780a7ee7730f  init.o.after.asm

 [ bp: Massage. ]
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lkml.kernel.org/r/20200928100004.25674-1-lukas.bulwahn@gmail.com

arch/x86/mm/init.c

@@ -596,7 +596,7 @@ static unsigned long __init get_new_step_size(unsigned long step_size)
 static void __init memory_map_top_down(unsigned long map_start,
 				       unsigned long map_end)
 {
-	unsigned long real_end, start, last_start;
+	unsigned long real_end, last_start;
 	unsigned long step_size;
 	unsigned long addr;
 	unsigned long mapped_ram_size = 0;
@@ -609,7 +609,7 @@ static void __init memory_map_top_down(unsigned long map_start,
 	step_size = PMD_SIZE;
 	max_pfn_mapped = 0; /* will get exact value next */
 	min_pfn_mapped = real_end >> PAGE_SHIFT;
-	last_start = start = real_end;
+	last_start = real_end;
 
 	/*
 	 * We start from the top (end of memory) and go to the bottom.
@@ -618,6 +618,8 @@ static void __init memory_map_top_down(unsigned long map_start,
 	 * for page table.
 	 */
 	while (last_start > map_start) {
+		unsigned long start;
+
 		if (last_start > step_size) {
 			start = round_down(last_start - 1, step_size);
 			if (start < map_start)