Hold the proc lock while calling proc_sstep() since the function asserts

it and remove a PRELE() that didn't have a matching PHOLD().  The calling
code already has a PHOLD anyway.

MFC after:	1 week

sys/fs/procfs/procfs_ctl.c

@@ -245,9 +245,8 @@ out:
 	 * What does it mean to single step a threaded program?
 	 */
 	case PROCFS_CTL_STEP:
-		PROC_UNLOCK(p);
 		error = proc_sstep(FIRST_THREAD_IN_PROC(p)); /* XXXKSE */
-		PRELE(p);
+		PROC_UNLOCK(p);
 		if (error)
 			return (error);
 		break;