Hyper-V: storvsc: Enhance srb_status code handling.

In hv_storvsc_io_request() when coring, prevent changing of the send channel
from the base channel to another one. storvsc_poll always probes on the base
channel.

Based upon conversations with Microsoft, changed the handling of srb_status
codes. Most we should never get, others yes. All are treated as retry-able
except for two. We should not get these statuses, but if we ever do, the I/O
state is not known.

Submitted by: Alexander Sideropoulos <Alexander.Sideropoulos@netapp.com>
Reviewed by: trasz, allanjude, whu
MFC after: 1 week
Sponsored by: Netapp Inc
Differential Revision: https://reviews.freebsd.org/D25756