x86/efi: Test for EFI_MEMMAP functionality when iterating EFI memmap

x86/efi: Test for EFI_MEMMAP functionality when iterating EFI memmap

Both efi_find_mirror() and efi_fake_memmap() really want to know
whether the EFI memory map is available, not just whether the machine
was booted using EFI. efi_fake_memmap() even has a check for
EFI_MEMMAP at the start of the function.

Since we've already got other code that has this dependency, merge
everything under one if() conditional, and remove the now superfluous
check from efi_fake_memmap().

Tested-by: Dave Young <dyoung@redhat.com> [kexec/kdump]
Tested-by: Ard Biesheuvel <ard.biesheuvel@linaro.org> [arm]
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Taku Izumi <izumi.taku@jp.fujitsu.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Xishi Qiu <qiuxishi@huawei.com>
Cc: Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Matt Fleming <matt@codeblueprint.co.uk>