SELinux: Make selinux_kernel_create_files_as() shouldn't just always return 0

Make selinux_kernel_create_files_as() return an error when it gets one, rather
than unconditionally returning 0.

Without this, cachefiles doesn't return an error if the SELinux policy doesn't
let it create files with the label of the directory at the base of the cache.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>