RISC-V: Fix up a cherry-pick warning in setup_vm_final()

This triggers a -Wdeclaration-after-statement as the code has changed a
bit since upstream.  It might be better to hoist the whole block up, but
this is a smaller change so I went with it.

arch/riscv/mm/init.c:755:16: warning: mixing declarations and code is a C99 extension [-Wdeclaration-after-statement]
             unsigned long idx = pgd_index(__fix_to_virt(FIX_FDT));
                           ^
     1 warning generated.

Fixes: bbf94b0 ("riscv: Move early dtb mapping into the fixmap region")
Reported-by: kernel test robot <[email protected]>
Link: https://lore.kernel.org/oe-kbuild-all/[email protected]/
Signed-off-by: Palmer Dabbelt <[email protected]>
Signed-off-by: Alexandre Ghiti <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

arch/riscv/mm/init.c

@@ -714,6 +714,7 @@ static void __init setup_vm_final(void)
 {
 	uintptr_t va, map_size;
 	phys_addr_t pa, start, end;
+	unsigned long idx __maybe_unused;
 	u64 i;
 
 	/**
@@ -733,7 +734,7 @@ static void __init setup_vm_final(void)
 	 * directly in swapper_pg_dir in addition to the pgd entry that points
 	 * to fixmap_pte.
 	 */
-	unsigned long idx = pgd_index(__fix_to_virt(FIX_FDT));
+	idx = pgd_index(__fix_to_virt(FIX_FDT));
 
 	set_pgd(&swapper_pg_dir[idx], early_pg_dir[idx]);
 #endif