depmod: Write index in pre-order

depmod: Write index in pre-order

Index files in pre-order have a significant performance improvement
for libkmod users.

On Arch Linux system, dumping configuration takes 296 read calls
in pre-order, compared to 4080 in post-order. Tests on a Raspberry
Pi 2 test system have shown an improvement by 9 %.

Even writing is faster now. This happens because we must know in
advance how many bytes index nodes will consume in the resulting file.
Although this code calculates it on the fly and caches the results,
saving lseek system calls has a significant positive effect which
compensates this extra overhead.

On Arch Linux system, writing index takes 6143 lseek calls now, compared
to previous 83701, leading to a performance gain of 13 %.

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>
Link: https://github.com/kmod-project/kmod/pull/188
Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>