SELinux: call cap_file_mmap in selinux_file_mmap

Currently SELinux does not check CAP_SYS_RAWIO in the file_mmap hook.  This
means there is no DAC check on the ability to mmap low addresses in the
memory space.  This function adds the DAC check for CAP_SYS_RAWIO while
maintaining the selinux check on mmap_zero.  This means that processes
which need to mmap low memory will need CAP_SYS_RAWIO and mmap_zero but will
NOT need the SELinux sys_rawio capability.

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