efi_loader: overflow in efi_allocate_pages

On 32bit systems (pages << EFI_PAGE_SHIFT) may lead to an overflow which
does not occur in 64bit arithmetics.

An overflow of (pages << EFI_PAGE_SHIFT) on 64bit systems should be treated
as an error.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>