nds32: fix up stack guard gap

Commit 1be7107 ("mm: larger stack guard gap, between vmas") fixed
up all architectures to deal with the stack guard gap.  But when nds32
was added to the tree, it forgot to do the same thing.

Resolve this by properly fixing up the nsd32's version of
arch_get_unmapped_area()

Cc: Nick Hu <[email protected]>
Cc: Greentime Hu <[email protected]>
Cc: Vincent Chen <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Qiang Liu <[email protected]>
Cc: stable <[email protected]>
Reported-by: iLifetruth <[email protected]>
Acked-by: Hugh Dickins <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

arch/nds32/mm/mmap.c

@@ -59,7 +59,7 @@ arch_get_unmapped_area(struct file *filp, unsigned long addr,
 
 		vma = find_vma(mm, addr);
 		if (TASK_SIZE - len >= addr &&
-		    (!vma || addr + len <= vma->vm_start))
+		    (!vma || addr + len <= vm_start_gap(vma)))
 			return addr;
 	}