Avoid a cache miss accessing an mbuf ext_pgs pointer when doing SW kTLS.

For a Netflix 90Gb/s 100% TLS software kTLS workload, this reduces
the CPI of tcp_m_copym() from ~3.5 to ~2.5 as reported by vtune.

Reviewed by: jtl, rrs
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D23998