mm/nommu: switch do_mmap_private to kernel_read

Instead of playing with the address limit.  This also gains us
validation of the kvec and proper atime updates.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>

mm/nommu.c

@@ -1164,17 +1164,12 @@ static int do_mmap_private(struct vm_area_struct *vma,
 
 	if (vma->vm_file) {
 		/* read the contents of a file into the copy */
-		mm_segment_t old_fs;
 		loff_t fpos;
 
 		fpos = vma->vm_pgoff;
 		fpos <<= PAGE_SHIFT;
 
-		old_fs = get_fs();
-		set_fs(KERNEL_DS);
-		ret = __vfs_read(vma->vm_file, base, len, &fpos);
-		set_fs(old_fs);
-
+		ret = kernel_read(vma->vm_file, base, len, &fpos);
 		if (ret < 0)
 			goto error_free;