MFC 306072-306074,306076

MFC 306072-306074,306076

306072
    hyperv/vmbus: Allow bufrings preallocation.

    The assumption that the channel is only opened upon synthetic device
    attach time no longer holds, e.g. Hyper-V network device MTU changes.
    We have to allow device drivers to preallocate bufrings, e.g. in
    attach DEVMETHOD, to prevent bufring allocation failure once the
    system memory is fragmented after running for a while.

    Sponsored by:   Microsoft
    Differential Revision:  https://reviews.freebsd.org/D7960

306073
    hyperv/hn: Allocate bufrings in attach DEVMETHOD.

    So that reinitialization, e.g. MTU change, will not fail when the system
    memory is excessively fragmented.

    Sponsored by:   Microsoft
    Differential Revision:  https://reviews.freebsd.org/D7961

306074
    hyperv/vmbus: Assert that the bufring address is page aligned.

    Sponsored by:   Microsoft
    Differential Revision:  https://reviews.freebsd.org/D7962

306076
    hyperv/hn: Put debug messages under bootverbose

    While I'm here, strip blank line.

    Sponsored by:   Microsoft
    Differential Revision:  https://reviews.freebsd.org/D7963

git-svn-id: svn://svn.freebsd.org/base/stable/10@307247 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f