Fix sosend() for the case where mbufs are passed in while doing ktls.

For kernel tls, sosend() needs to call ktls_frame() on the mbuf list
to be sent. Without this patch, this was only done when sosend()'s
arguments used a uio_iov and not when an mbuf list is passed in.
At this time, sosend() is never called with an mbuf list argument when
kernel tls is in use, but will be once nfs-over-tls has been incorporated
into head.

Reviewed by: gallatin, glebius
Differential Revision: https://reviews.freebsd.org/D24674