net/9p: Add __force to cast of __user pointer

A recent commit that removed unnecessary casts of pointers
to the same type uncovered a missing __force cast.

Add it.

Reported by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: NJoe Perches <joe@perches.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/9p/client.c

@@ -1548,7 +1548,7 @@ p9_client_read(struct p9_fid *fid, char *data, char __user *udata, u64 offset,
 			kernel_buf = 1;
 			indata = data;
 		} else
-			indata = udata;
+			indata = (__force char *)udata;
 		/*
 		 * response header len is 11
 		 * PDU Header(7) + IO Size (4)