nommu: fix do_munmap() error path

When removing a VMA from the tree fails due to no memory, do not free the
VMA since a reference still exists.

Link: https://lkml.kernel.org/r/20230109205708.956103-1-Liam.Howlett@oracle.com
Fixes: 8220543 ("nommu: remove uses of VMA linked list")
Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Yu Zhao <yuzhao@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/nommu.c

@@ -1509,7 +1509,8 @@ int do_munmap(struct mm_struct *mm, unsigned long start, size_t len, struct list
 erase_whole_vma:
 	if (delete_vma_from_mm(vma))
 		ret = -ENOMEM;
-	delete_vma(mm, vma);
+	else
+		delete_vma(mm, vma);
 	return ret;
 }