MFC 295302,295303

MFC 295302,295303

295302
    hyperv/hn: Factor out hn_encap() from hn_start_locked()

    It will be shared w/ upcoming ifnet.if_transmit implementaion.

    No functional changes.

    Reviewed by:        adrian
    Approved by:        adrian (mentor)
    MFC after:  1 week
    Sponsored by:       Microsoft OSTC
    Differential Revision:      https://reviews.freebsd.org/D5158

295303
    hyperv/hn: Recover half of the chimney sending space

    We lost half of the chimney sending space, because we mis-used
    ffs() on a 64 bits mask, where ffsl() should be used.

    While I'm here:
    - Use system atomic operation instead.
    - Stringent chimney sending index assertion.

    Reviewed by:        adrian
    Approved by:        adrian (mentor)
    MFC after:  1 week
    Sponsored by:       Microsoft OSTC
    Differential Revision:      https://reviews.freebsd.org/D5159

git-svn-id: svn://svn.freebsd.org/base/stable/10@301857 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f