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: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/9p/protocol.c b/net/9p/protocol.c
index 9ee48cb..3d33ecf 100644
--- a/net/9p/protocol.c
+++ b/net/9p/protocol.c
@@ -368,7 +368,7 @@
 				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,