SELinux: security_read_policy should take a size_t not ssize_t

The len should be an size_t but is a ssize_t.  Easy enough fix to silence
build warnings.  We have no need for signed-ness.

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

security/selinux/ss/services.c

@@ -3189,7 +3189,7 @@ out:
  * @len: length of data in bytes
  *
  */
-int security_read_policy(void **data, ssize_t *len)
+int security_read_policy(void **data, size_t *len)
 {
 	int rc;
 	struct policy_file fp;