nbd/server: fix NBD_CMD_CACHE

nbd/server: fix NBD_CMD_CACHE

We should not go to structured-read branch on CACHE command, fix that.

Bug introduced in bc37b06a5cde24 "nbd/server: introduce NBD_CMD_CACHE"
with the whole feature and affects 3.0.0 release.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
CC: qemu-stable@nongnu.org
Message-Id: <20181003144738.70670-1-vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[eblake: commit message typo fix]
Signed-off-by: Eric Blake <eblake@redhat.com>
(cherry picked from commit 2f454defc23e1be78f2a96bad2877ce7829f61b4)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>