IfAPI: Add needed APIs for mbuf support

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.

Reviewed By: jhb
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D38340