Fix cleanup race between unp_dispose and unp_gc

Fix cleanup race between unp_dispose and unp_gc

unp_dispose and unp_gc could race to teardown the same mbuf chains, which
can lead to dereferencing freed filedesc pointers.

This patch adds an IGNORE_RIGHTS flag on unpcbs marking the unpcb's RIGHTS
as invalid/freed. The flag is protected by UNP_LIST_LOCK.

To serialize against unp_gc, unp_dispose needs the socket object. Change the
dom_dispose() KPI to take a socket object instead of an mbuf chain directly.

PR:		194264
Differential Revision:	https://reviews.freebsd.org/D3044
Reviewed by:	mjg (earlier version)
Approved by:	markj (mentor)
Obtained from:	mjg
MFC after:	1 month
Sponsored by:	EMC / Isilon Storage Division