accept_filter: Fix filter parameter handling

For filters which implement accf_create, the setsockopt(2) handler
caches the filter name in the socket, but it also incorrectly frees the
buffer containing the copy, leaving a dangling pointer.  Note that no
accept filters provided in the base system are susceptible to this, as
they don't implement accf_create.

Reported by:	Alexey Kulaev <alex.qart@gmail.com>
Discussed with:	emaste
Security:	kernel use-after-free
MFC after:	3 days
Sponsored by:	The FreeBSD Foundation

sys/kern/uipc_accf.c

@@ -299,6 +299,7 @@ accept_filt_setopt(struct socket *so, struct sockopt *sopt)
 	so->sol_accept_filter = afp;
 	so->sol_accept_filter_arg = accept_filter_arg;
 	so->sol_accept_filter_str = accept_filter_str;
+	accept_filter_str = NULL;
 	so->so_options |= SO_ACCEPTFILTER;
 out:
 	SOCK_UNLOCK(so);