[PATCH] pipe.c: don't write to readonly filesystems

The pipe code's mtime and ctime updates are causing writes to read-only
mounted filesystems.

Fix that up by teaching inode_update_time() to honour readonly mounts, and
call it from the pipe code.

fs/inode.c

@@ -1148,9 +1148,14 @@ void update_atime(struct inode *inode)
 
 void inode_update_time(struct inode *inode, int ctime_too)
 {
-	struct timespec now = current_kernel_time();
+	struct timespec now;
 	int sync_it = 0;
 
+	if (IS_RDONLY(inode))
+		return;
+
+	now = current_kernel_time();
+
 	if (inode_times_differ(inode, &inode->i_mtime, &now))
 		sync_it = 1;
 	inode->i_mtime = now;

fs/pipe.c

@@ -208,10 +208,8 @@ pipe_write(struct file *filp, const char __user *buf, size_t count, loff_t *ppos
 		wake_up_interruptible(PIPE_WAIT(*inode));
 		kill_fasync(PIPE_FASYNC_READERS(*inode), SIGIO, POLL_IN);
 	}
-	if (ret > 0) {
-		inode->i_ctime = inode->i_mtime = CURRENT_TIME;
-		mark_inode_dirty(inode);
-	}
+	if (ret > 0)
+		inode_update_time(inode, 1);	/* mtime and ctime */
 	return ret;
 }