Security: Fix the comment of cap_file_mmap()

In the comment of cap_file_mmap(), replace mmap_min_addr to be dac_mmap_min_addr.

Signed-off-by: Zhitong Wang <zhitong.wangzt@alibaba-inc.com>
Acked-by: Eric Paris <eparis@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>