hyperv/hn: Avoid duplicate csum features settings

• Record csum features in softc, so we don't need to duplicate the logic from attach path to ioctl path.
• Protect if_capenable and if_hwassist changes by main lock.
• Prefer turn on/off bits in if_hwassist explicitly instead of using XOR.

Reviewed by: adrian, Hongjiang Zhang <honzhan microsoft com>
Approved by: adrian (mentor)
MFC after: 1 week
Sponsored by: Microsoft OSTC
Differential Revision: https://reviews.freebsd.org/D5085