libkmod: store common section off/size and use them

libkmod: store common section off/size and use them

Currently, we repeatedly loop over the elf headers looking for five well
known sections. Just do it once in kmod_elf_new() and reuse the data as
needed.

Note that not all sections are guaranteed to be available, so check and
ELFDBG print the ones which are missing.

v2: quit looping when found, ELFDBG print missing sections
v3: match the first section name, use a loop

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Tobias Stoeckmann <tobias@stoeckmann.org>
Link: https://github.com/kmod-project/kmod/pull/210
Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>