fix sosend() so that it can handle an mbuf list for KTLS

[rmacklem]

Summary

When the nfs-over-tls krpc calls sosend() with a list of ext_pgs
mbufsto be handled via the ktls, ktls_frame() needs to be
called on them.

This patch just does the same call as the case where the data
has been copied in via m_uiotombuf() below where the patch
is applied.

I'll admit I don't understand if/when tls_rtype is not TLS_RLTYPE_APP,
but I did what the m_uiotombuf() case already does.

Test Plan

It has seen minimal testing using nfs-over-tls sofar,
but I will be doing further testing soon.

[glebius]

This looks correct to me.