Clean up iflib sysctls

Remove sysctls:
txq_drain_encapfail - now a duplicate of encap_txd_encap_fail
intr_link - was never incremented
intr_msix - was never incremented
rx_zero_len - was never incremented

The following were not incremented in all code-paths that apply:
m_pullups, mbuf_defrag, rxd_flush, tx_encap, rx_intr_enables, tx_frees,
encap_txd_encap_fail.

Fixes:
Replace the broken collapse_pkthdr() implementation with an MPASS().
fl_refills and fl_refills_large were not incremented when using netmap.

Reviewed by: gallatin
Approved by: re (marius)
Sponsored by: Limelight Networks
Differential Revision: https://reviews.freebsd.org/D16733