efi_loader: correcty determine total device path length

Device paths may consist of multiple instances. Up to now we have only
considered the size of the first instance. For the services of the
EFI_DEVICE_PATH_UTILITIES_PROTOCOL in most cases the total length of the
device path is relevant.

So let's rename efi_dp_size() to efi_dp_instance_size() and create a new
function efi_dp_size() that calculates the total device path length.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>