This change updates the netlink message handling for the infra link on
the POSIX platform to be more robust and efficient for `RTM_NEWLINK`
and `RTM_DELLINK` messages.
The logic for handling infra index changes is:
1. On `RTM_DELLINK`, the code verifies `ifinfo->ifi_index` is for the
currently infra index `mInfraIfIndex`.
2. On `RTM_NEWLINK`, it identifies the infra by its name
`if_indextoname(ifinfo->ifi_index)` and updates `mInfraIfIndex`.
Yang Song
61e2f2877d