block/nfs: refuse readahead if cache.direct is on

block/nfs: refuse readahead if cache.direct is on

if we open a NFS export with disabled cache we should refuse
the readahead feature as it will cache data inside libnfs.

If a export was opened with readahead enabled it should
futher not be allowed to disable the cache while running.

Cc: qemu-stable@nongnu.org
Signed-off-by: Peter Lieven <pl@kamp.de>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Message-id: 1463662083-20814-2-git-send-email-pl@kamp.de
Signed-off-by: Jeff Cody <jcody@redhat.com>
(cherry picked from commit 38f8d5e0251ae7d8257cf099cb3e5a375ef60378)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>