IfAPI: Add needed APIs for mbuf support

[jhibbits]

Summary

Add 2 new APIs for supporting recent mbuf changes:

• 36e0a362ac added the m_snd_tag_alloc() wrapper around if_snd_tag_alloc(). Push this down to the ifnet level.
• 4d7a1361ef adds the m_rcvif_serialize()/m_rcvif_restore() KPIs to serialize and restore an ifnet pointer. Add the necessary wrapper to get the index generation for this.

[jhibbits]

sys/kern/kern_mbuf.c

1589

Not sure if it's better to just flat remove m_snd_tag_alloc() in this case. Thoughts?

[jhb]

sys/kern/kern_mbuf.c

1589

It might very well be. I would perhaps do that as a separate followup though after this commit.