fix sosend_generic() so that it can handle a list of ext_pgs mbufs post r359919

[rmacklem]

Summary

r359919 modified "struct mbuf" so that a ext_pgs mbuf could not have
a pkthdr. As such, when I list of ext_pgs mbufs is passed into sosend_generic,
it would fail, since it would try and use a field as m_pkthdr.len (usually 0 fortunately).

This little patch adds code to check for M_PKTHDR and calculates the length
via m_length() if not.

Test Plan

This has been tested with the code in base/projects/nfs-over-tls, which
generates the NFS RPC messages in a list of ext_pgs mbufs when
KERN_TLS will be used to send them.

Since there are no cases where sosend_generic() is called with a
list of ext_pgs mbufs in base/head, the above should be the only
use case for now.