efi: vars: prohibit reading random seed variables

In anticipation of putting random seeds in EFI variables, it's important
that the random GUID namespace of variables remains hidden from
userspace. We accomplish this by not populating efivarfs with entries
from that GUID, as well as denying the creation of new ones in that
GUID.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>