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