vfs: explicitly cast s_maxbytes in fiemap_check_ranges

If fiemap_check_ranges is passed a large enough value, then it's
possible that the value would be cast to a signed value for comparison
against s_maxbytes when we change it to loff_t. Make sure that doesn't
happen by explicitly casting s_maxbytes to an unsigned value for the
purposes of comparison.

Signed-off-by: Jeff Layton <[e-mail]>
Cc: Christoph Hellwig <[e-mail]>
Cc: Robert Love <[e-mail]>
Cc: Al Viro <[e-mail]>
Cc: Johannes Weiner <[e-mail]>
Cc: Mandeep Singh Baines <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Al Viro <[e-mail]>

fs/ioctl.c

···
 static int fiemap_check_ranges(struct super_block *sb,
 			       u64 start, u64 len, u64 *new_len)
 {
+	u64 maxbytes = (u64) sb->s_maxbytes;
+
 	*new_len = len;
 
 	if (len == 0)
 		return -EINVAL;
 
-	if (start > sb->s_maxbytes)
+	if (start > maxbytes)
 		return -EFBIG;
 
 	/*
 	 * Shrink request scope to what the fs can actually handle.
 	 */
-	if ((len > sb->s_maxbytes) ||
-	    (sb->s_maxbytes - len) < start)
-		*new_len = sb->s_maxbytes - start;
+	if (len > maxbytes || (maxbytes - len) < start)
+		*new_len = maxbytes - start;
 
 	return 0;
 }