Fix divergence in attach_interfaces extensions

I pointed out in revision 7 of Ibe4cbd025700aa0d73a6d4d19fbf8f0005509428
that it we shouldn't use a different variable name in the identical code
shared between the v3 and v2 extensions, but that was never fixed before
merging.

Change-Id: I7c2853db2cbad30a37efd98947ebe687b37bfbe8
Merge "Fix divergence in attach_interfaces extensions"