efi_loader: fix bug in efi_get_memory_map

When booting shim -> fallback -> shim -> grub -> linux the memory map is
a bit larger than the size linux passes in on the first call.  But in
the EFI_BUFFER_TOO_SMALL case we were not passing back the updated size
to linux so it would loop forever.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Alexander Graf <agraf@suse.de>