9p: virtio: make sure 'offs' is initialized in zc_request

Similarly to the previous patch: offs can be used in handle_rerrors
without initializing on small payloads; in this case handle_rerrors will
not use it because of the size check, but it doesn't hurt to make sure
it is zero to please scan-build.

This fixes the following warning:
net/9p/trans_virtio.c:539:3: warning: 3rd function call argument is an uninitialized value [core.CallAndMessage]
handle_rerror(req, in_hdr_len, offs, in_pages);
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Reviewed-by: Simon Horman <[e-mail]>
Signed-off-by: Dominique Martinet <[e-mail]>
Signed-off-by: Eric Van Hensbergen <[e-mail]>

net/9p/trans_virtio.c

···
 	struct page **in_pages = NULL, **out_pages = NULL;
 	struct virtio_chan *chan = client->trans;
 	struct scatterlist *sgs[4];
-	size_t offs;
+	size_t offs = 0;
 	int need_drop = 0;
 	int kicked = 0;