Fix CARP echo back for ESX vSwitch

[slw_zxy.spb.ru]

Summary

The 8151740c881f1f400dab7a3f4c7bb23439c11b66 commit introduces a workaround for the vSwitch bug, where an interface accepts a sent back multicast announcement. And that doesn't work at the moment:
IFNET_FOREACH_IFA iterates only addresses with non-zero ifa_carp, and the packet is sent with ifa_carp == NULL (i.e. from a general, non-carp source).
Also, it is checked that the carp_vhid field in the received packet is 0, but vhid should be from 1 to 255.

[melifaro]

I'd rather do it slightly differently - there is already carp_forus_p() hook that is called in ether_input_internal(). I'd extend it to pass the source MAC there as well so the routine can check if this is the looped packet.
If the latter is true, I'd stamp mbuf with either M_LOOP or M_PROMISC flag to ensure it will be dropped early enough.