mm/mmap: fix obsolete comment of find_extend_vma

mmget_still_valid() has already been removed via commit 4d45e75 ("mm:
remove the now-unnecessary mmget_still_valid() hack").  Update the
corresponding comment.

Link: https://lkml.kernel.org/r/20220709092527.47778-1-linmiaohe@huawei.com
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/mmap.c

@@ -2541,7 +2541,6 @@ find_extend_vma(struct mm_struct *mm, unsigned long addr)
 	vma = find_vma_prev(mm, addr, &prev);
 	if (vma && (vma->vm_start <= addr))
 		return vma;
-	/* don't alter vm_end if the coredump is running */
 	if (!prev || expand_stack(prev, addr))
 		return NULL;
 	if (prev->vm_flags & VM_LOCKED)