MFC r218757:

MFC r218757:

  Mfp4 CH=177274,177280,177284-177285,177297,177324-177325

    VNET socket push back:
    try to minimize the number of places where we have to switch vnets
    and narrow down the time we stay switched.  Add assertions to the
    socket code to catch possibly unset vnets as seen in r204147.

    While this reduces the number of vnet recursion in some places like
    NFS, POSIX local sockets and some netgraph, .. recursions are
    impossible to fix.

    The current expectations are documented at the beginning of
    uipc_socket.c along with the other information there.

    Sponsored by: The FreeBSD Foundation
    Sponsored by: CK Software GmbH
    Reviewed by:  jhb
    Tested by:    zec

  Tested by:    Mikolaj Golub (to.my.trociny gmail.com)

git-svn-id: svn://svn.freebsd.org/base/stable/8@220733 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f