abidiff: Fix handling of linux-kernel-mode

abidiff: Fix handling of linux-kernel-mode

This fixes 08e76022 ("Support Linux Kernel ABI whitelist files") and
8fd02e0a ("Use the CTF reader by default when applicable").

The first commit removed handling of option --no-linux-kernel-mode. The
second one stopped passing opts.linux_kernel_mode down to the library
which also caused changing the default linux-kernel-mode of abidiff from
true to false. Both changes look unintentional as they are not mentioned
in either commit message.

Restore handling of the linux-kernel-mode in abidiff to match what is
described in doc/manuals/abidiff.rst and make it again consistent with
abidw.

	* tools/abidiff.cc (parse_command_line): Recognize
	  --no-linux-kernel-mode.
	  (main): Pass opts.linux_kernel_mode down to the library.

Signed-off-by: Petr Pavlu <petr.pavlu@suse.com>
Signed-off-by: Dodji Seketeli <dodji@redhat.com>