Handle HWMP if_transmit() failure gracefully.
Handle HWMP if_transmit() failure gracefully.

If if_transmit() fails, the node ref may need freeing.

This is based on the same logic used by the ageq, which the mesh code
(re) uses for frames which need to be staged before transmitting.
It also does the same thing - if M_ENCAP is set on the mbuf, it treats
the recvif pointer as a node reference and derefs it.

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