As vhost scsi device struct is large, if the device is
created on a busy system, kzalloc() might fail, so this patch does a
fallback to vzalloc().

As vmalloc() adds overhead on data-path, add __GFP_REPEAT
to kzalloc() flags to do this fallback only when really needed.
Reviewed-by: NAsias He <asias@redhat.com>
Reported-by: NDan Aloni <alonid@stratoscale.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>