fix mmap() return value to conform to POSIX

POSIX said:
mmap() should return [EOVERFLOW] if the file is a regular file and the value of off + len exceeds the offset maximum established in the open file description associated with fildes.

@@ -906,7 +906,7 @@ unsigned long do_mmap_pgoff(struct file
        

	/* offset overflow? */
        if ((pgoff + (len >> PAGE_SHIFT)) < pgoff)

- return -EINVAL;
+ return -EOVERFLOW;

        /* Too many mappings? */
        if (mm->map_count > sysctl_max_map_count)