livepatch: Small shadow variable documentation fixes

The description of the basic operations was a bit inconsistent
and based on older version of the patchset.

Also the size of the spinlock structure should be allocated
instead of the pointer.

Signed-off-by: Petr Mladek <pmladek@suse.com>
Acked-by: Joe Lawrence <joe.lawrence@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>