ng_eiface: switch VNETs when injecting mbufs into netgraph

A ng_eiface instance may be on lease to a different vnet while
remaining tied to its parent vnet. In such circumstances, before
injecting mbufs into netgraph, curvnet must be set to that of the
ng_eiface's netgraph node. Mark the vnet transition as QUIET,
since otherwise it would be recorded as a curvnet recursion when
ng_eiface's ifnet resides in the same (parent) vnet as its
netgraph node.

PR: 274028
Reported by: Dancho Penev <dpslavov@hotmail.com>
MFC after: 1 week