9p: use unsigned integers for nwqid/count

As specification says, all integers in messages are unsigned. Let's fix
behaviour of p9pdu_vreadf()/p9pdu_vwritef() accordingly.

Fix for p9pdu_vreadf() is critical. If server replies with Rwalk, where
nwqid > SHRT_MAX, the value will be interpreted as negative. kmalloc, in
its order, will cast the value to (very big) size_t.

It should never happen in normal situation: we never submit Twalk with
nwname > 16, but malicious or broken server can still produce
problematic Rwalk.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>