Security: allow capable check to permit mmap or low vm space

On a kernel with CONFIG_SECURITY but without an LSM which implements
security_file_mmap it is impossible for an application to mmap addresses
lower than mmap_min_addr.  Based on a suggestion from a developer in the
openwall community this patch adds a check for CAP_SYS_RAWIO.  It is
assumed that any process with this capability can harm the system a lot
more easily than writing some stuff on the zero page and then trying to
get the kernel to trip over itself.  It also means that programs like X
on i686 which use vm86 emulation can work even with mmap_min_addr set.

Signed-off-by: Eric Paris <eparis@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>