libkmod: Do not inititialize file->memory on open

libkmod: Do not inititialize file->memory on open

Add a separate function to load the file contents when it's needed.
When it's not needed on the path of loading modules via finit_module(),
there is no need to mmap the file. This will help support loading
modules with the in-kernel compression support.

This is done differently than the lazy initialization for
kmod_file_get_elf() because on the contents case there is also the
file->size to be updated. It would be a weird API to return the pointer
and have the size changed as a side-effect.

Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>