bridge: Get rid of "port_ifidx" in struct iface. Fix bonding hash.

bridge: Get rid of "port_ifidx" in struct iface.  Fix bonding hash.

This is a first step toward changing the array of ifaces in struct port
to a more suitable data structure.

As a side effect this fixes a bonding problem that I noticed via code
inspection.  Before this commit, each bond_entry specified an interface
via index.  If an iface was deleted, however, this shifted all of the
iface indexes, and the code didn't compensate for that.  This commit fixes
the problem by using pointers to ifaces instead, which don't shift around.