livepatch: update MAINTAINERS

Update MAINTAINERS for livepatching to reflect status quo better.

Also move the tree to a shared location, as we are moving more
towards group maintainership model.

Acked-by: Miroslav Benes <mbenes@suse.cz>
Acked-by: Jessica Yu <jeyu@kernel.org>
Acked-by: Petr Mladek <pmladek@suse.com>
Acked-by: Joe Lawrence <joe.lawrence@redhat.com>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>