drivers: char: mem: fix IS_ERROR_VALUE usage

IS_ERR_VALUE macro should be used only with unsigned long type.
Specifically it works incorrectly with longer types.

The patch follows conclusion from discussion on LKML [1][2].

[1]: http://permalink.gmane.org/gmane.linux.kernel/2120927
[2]: http://permalink.gmane.org/gmane.linux.kernel/2150581

Signed-off-by: Andrzej Hajda <[e-mail]>
Acked-by: Al Viro <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/char/mem.c

···
 		offset += file->f_pos;
 	case SEEK_SET:
 		/* to avoid userland mistaking f_pos=-9 as -EBADF=-9 */
-		if (IS_ERR_VALUE((unsigned long long)offset)) {
+		if ((unsigned long long)offset >= -MAX_ERRNO) {
 			ret = -EOVERFLOW;
 			break;
 		}