bridge: Improve name and comments for bond_entry's "iface_tag" member.

bridge: Improve name and comments for bond_entry's "iface_tag" member.

The iface_tag name and comment implied that it was really just a copy of
the 'tag' member of struct iface, but in fact it has a completely different
purpose: it represents the binding of a bond_entry to a particular iface.
It is invalidated when the bond_entry has to be redirected to a different
iface, not when the iface itself changes.  I hope that this commit helps
to clarify.