hyperv: Use atomic_fetchadd_int to get GPADL id.

[sepherosa_gmail.com]

Summary

Read then atomic_add_int is not MPSAFE.

And don't decrease the global GPADL id upon GPADL establish messages allocation failure, which should be really rare (and we should convert the message allocation to use M_WAITOK); add comment about why we _must_ not decrease the global GPADL id there.

[honzhan_microsoft.com]

Yes, substract the next_gpadl_handle is risk here.