filemon_open: Don't record a process to trace here.

Only ioctl(FILEMON_SET_PID) should be setting the process-to-be-traced.

MFC after:	2 weeks
Sponsored by:	EMC / Isilon Storage Division

sys/dev/filemon/filemon.c

@@ -221,10 +221,9 @@ filemon_open(struct cdev *dev, int oflags __unused, int devtype __unused,
 		filemon = malloc(sizeof(struct filemon), M_FILEMON,
 		    M_WAITOK | M_ZERO);
 		sx_init(&filemon->lock, "filemon");
+		filemon->pid = -1;
 	}
 
-	filemon->pid = curproc->p_pid;
-
 	devfs_set_cdevpriv(filemon, filemon_dtr);
 
 	/* Get exclusive write access. */