9pfs: Fix warnings from Sparse

9pfs: Fix warnings from Sparse

Sparse report:

9pfs/virtio-9p.c:1953:9: warning: returning void-valued expression
9pfs/virtio-9p-handle.c:143:5: warning: returning void-valued expression
9pfs/virtio-9p-handle.c:160:5: warning: returning void-valued expression
9pfs/virtio-9p-local.c:384:5: warning: returning void-valued expression
9pfs/virtio-9p-local.c:415:5: warning: returning void-valued expression
9pfs/virtio-9p-proxy.c:672:5: warning: returning void-valued expression
9pfs/virtio-9p-proxy.c:689:5: warning: returning void-valued expression

Cc: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>