efi/libstub: Use cleanup helpers for freeing copies of the memory map

The EFI stub may obtain the memory map from the firmware numerous times,
and this involves doing a EFI pool allocation first, which needs to be
freed after use.

Streamline this using a cleanup helper, which makes the code easier to
follow.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>