net/9p: insulate the client against an invalid error code sent by a 9p server

A looney tunes server sending an invalid error code (which is !IS_ERR_VALUE)
can result in a client oops. So fix it by adding a check and converting unknown
or invalid error codes to -ESERVERFAULT.

Signed-off-by: Abhishek Kulkarni <adkulkar@umail.iu.edu>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>

net/9p/error.c

@@ -239,7 +239,7 @@ int p9_errstr2errno(char *errstr, int len)
 		errstr[len] = 0;
 		printk(KERN_ERR "%s: server reported unknown error %s\n",
 			__func__, errstr);
-		errno = 1;
+		errno = ESERVERFAULT;
 	}
 
 	return -errno;