Add documentation to kdump_ops

Describe the ops API.

Signed-off-by: Petr Tesarik <ptesarik@suse.cz>
Provide proper data cleanup

Many handlers allocate private data, which should be freed when the
kdump_ctx object is destroyed.

Signed-off-by: Petr Tesarik <ptesarik@suse.cz>
Fix memory cleanup in ELF error path

The new rules are simple - elf_probe owns the elfdump_priv structure.
When something fails, an error is returned, and elf_probe cleans up
all resources allocated so far.

Signed-off-by: Petr Tesarik <ptesarik@suse.cz>