Improve TX path of the VNIC driver

• Avoid memory leak when nicvf_tx_mbuf_locked() fails
• Introduce nicvf_xmit_locked() routine that uses drbr_peek(), drbr_advance() or drbr_putback() for a specific ifnet. This gives more clear and efficient design as well as prevents from dropping mbufs that where not sent due to temporary lack of descriptors.
• Add missing ETHER_BPF_MTAP() hook

Pointed out by: yongari
Reviewed by: wma
Obtained from: Semihalf
Sponsored by: Cavium
Differential Revision: https://reviews.freebsd.org/D5534