When detaching an unix domain socket, uipc_detach() checks

When detaching an unix domain socket, uipc_detach() checks
unp->unp_vnode pointer to detect if there is a vnode associated with
(binded to) this socket and does necessary cleanup if there is.

The issue is that after forced unmount this check may be too late as
the unp_vnode is reclaimed and the reference is stale.

To fix this provide a helper function that is called on a socket vnode
reclamation to do necessary cleanup.

Pointed by:	kib
Reviewed by:	kib
MFC after:	2 weeks