unix: use m_freemp() when disposing unix socket buffers

The new unix/dgram uses m_nextpkt linkage, while the old unix/stream
uses m_next linkage. This fixes memory leak.

Diagnosed by: khng
Reviewed by: khng, markj
PR: 279467
Fixes: 458f475df8e5912609c14208c189414a8255c738
Differential Revision: https://reviews.freebsd.org/D45478
MFC After: 1 week