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

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