hyperv/vmbus: Redefine SynIC message.

[sepherosa_gmail.com]

Summary

• Avoid unnecessary indirection.
• Avoid bit fields.
• Use __packed.

[howard0su_gmail.com]

sys/dev/hyperv/vmbus/hv_vmbus_drv_freebsd.c

131 ↗

(On Diff #17085)

volatile is not needed here since msg pointer will not change out of this context.

[sepherosa_gmail.com]

sys/dev/hyperv/vmbus/hv_vmbus_drv_freebsd.c

131 ↗

(On Diff #17085)

It will be updated by the host, so we need to make sure that compiler don't over-optimize the msg accessing.