efivars: adjust location of variable attribute

efivars: adjust location of variable attribute

I also thought about converting efi_set_variable() to use writev, but we don't
have loop_writev. I'm not sure if the loop around write here is important.
Coinceivably, it could make a difference it we were writing a long value.
The loop was introduced in b7749eb517ff5dd379cf61ee9fb50a0105ab2c0f, without
much comment unfortunately. So it doesn't seem worth the risk of changing this
to not use a loop, and writing loop_writev just for this also seems overkill.