This flag was added during the transition away from the legacy zone
allocator, commit c897b81311792ccf6a93feff2a405e2ae53f664e. The old
zone allocator effectively provided _NOFREE semantics, but I believe
they are not required for sockets. In particular, we use reference
counting to keep sockets live.

The one case which is kind of weird is sonewconn(), which returns a
pointer to a socket with reference count 0. This socket is still
effectively owned by the listening socket. Protocols must therefore be
careful to synchronize sonewconn() calls with their pru_close
implementations, since for listening sockets soclose() will abort the
child sockets. For example, TCP holds the listening socket's PCB read
locked across the sonewconn() call, which blocks tcp_usr_close().

Eliminating _NOFREE has several benefits: it enables use-after-free
detection (e.g., by KASAN) and lets the system reclaim memory from the
socket zone under memory pressure.

No functional change intended.