NFSD: Stricter buffer size checking in write_recoverydir()

While it's not likely a pathname will be longer than
SIMPLE_TRANSACTION_SIZE, we should be more careful about just
plopping it into the output buffer without bounds checking.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>

fs/nfsd/nfsctl.c

@@ -1260,8 +1260,9 @@ static ssize_t __write_recoverydir(struct file *file, char *buf, size_t size)
 
 		status = nfs4_reset_recoverydir(recdir);
 	}
-	sprintf(buf, "%s\n", nfs4_recoverydir());
-	return strlen(buf);
+
+	return scnprintf(buf, SIMPLE_TRANSACTION_LIMIT, "%s\n",
+							nfs4_recoverydir());
 }
 
 /**