nbd/client: Don't hard-disconnect on ESHUTDOWN from server

The NBD spec says that a server may fail any transmission request
with ESHUTDOWN when it is apparent that no further request from
the client can be successfully honored.  The client is supposed
to then initiate a soft shutdown (wait for all remaining in-flight
requests to be answered, then send NBD_CMD_DISC).  However, since
qemu's server never uses ESHUTDOWN errors, this code was mostly
untested since its introduction in commit b6f5d3b5.

More recently, I learned that nbdkit as the NBD server is able to
send ESHUTDOWN errors, so I finally tested this code, and noticed
that our client was special-casing ESHUTDOWN to cause a hard
shutdown (immediate disconnect, with no NBD_CMD_DISC), but only
if the server sends this error as a simple reply.  Further
investigation found that commit d2febedb introduced a regression
where structured replies behave differently than simple replies -
but that the structured reply behavior is more in line with the
spec (even if we still lack code in nbd-client.c to properly quit
sending further requests).  So this patch reverts the portion of
b6f5d3b5 that introduced an improper hard-disconnect special-case
at the lower level, and leaves the future enhancement of a nicer
soft-disconnect at the higher level for another day.

CC: qemu-stable@nongnu.org
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20171113194857.13933-1-eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
