9p: fix min_t() casting in p9pdu_vwritef()

I don't think we're actually likely to hit this limit but if we do
then the comparison should be done as size_t.  The original code
is equivalent to:
        len = strlen(sptr) % USHRT_MAX;
Signed-off-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/9p/protocol.c

@@ -368,7 +368,7 @@ p9pdu_vwritef(struct p9_fcall *pdu, int proto_version, const char *fmt,
 				const char *sptr = va_arg(ap, const char *);
 				uint16_t len = 0;
 				if (sptr)
-					len = min_t(uint16_t, strlen(sptr),
+					len = min_t(size_t, strlen(sptr),
 								USHRT_MAX);
 
 				errcode = p9pdu_writef(pdu, proto_version,