fix pgoff in "have to relocate" case of mremap()

Acked-by: NHugh Dickins <hugh.dickins@tiscali.co.uk>
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>

mm/mremap.c

@@ -495,7 +495,9 @@ unsigned long do_mremap(unsigned long addr,
 			map_flags |= MAP_SHARED;
 
 		new_addr = get_unmapped_area(vma->vm_file, 0, new_len,
-					vma->vm_pgoff, map_flags);
+					vma->vm_pgoff +
+					((addr - vma->vm_start) >> PAGE_SHIFT),
+					map_flags);
 		if (new_addr & ~PAGE_MASK) {
 			ret = new_addr;
 			goto out;