MFC: r347211

• Remove the unused ifc_link_irq and ifc_mtx_name members of struct iflib_ctx.
• Remove the only ever written to ift_db_mtx_name member of struct iflib_txq.
• Remove the unused or only ever written to ifr_size, ifr_cq_pidx, ifr_cq_gen and ifr_lro_enabled members of struct iflib_rxq.
• Consistently spell DMA, RX and TX uppercase in comments, messages etc. instead of mixing with some lowercase variants.
• Consistently use if_t instead of a mix of if_t and struct ifnet pointers.
• Bring the function comments of _iflib_fl_refill(), iflib_rx_sds_free() and iflib_fl_setup() in line with reality.
• Judging problem reports, people are wondering what on earth messages like: "TX(0) desc avail = 1024, pidx = 0" are trying to indicate. Thus, extend this string to be more like that of non-iflib(4) Ethernet MAC drivers, notifying about a watchdog timeout due to which the interface will be reset.
• Take advantage of the M_HAS_VLANTAG macro.
• Use false/true rather than FALSE/TRUE for variables of type bool.
• Use FALLTHROUGH as advocated by style(9).