accept_filter: Fix filter parameter handling

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

(cherry picked from commit 653a437c04440495cd8e7712c7cf39444f26f1ee)