zpool: flush output before sleeping

Several zpool commands (status, list, iostat) have modes that present
some information, sleep a while, present the current state, sleep, etc.
Some of those had ways to invoke them that when piped would appear to do
nothing for a while, because non-terminals are block-buffered, not
line-buffered, by default.  Fix this by forcing a flush before sleeping.

In particular, all of these buffered:
- zpool status <pool> <interval>
- zpool iostat -y<m> <pool> <interval>
- zpool list <pool> <interval>

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes openzfs#15593

cmd/zpool/zpool_main.c

@@ -5950,6 +5950,7 @@ zpool_do_iostat(int argc, char **argv)
 				print_iostat_header(&cb);
 
 			if (skip) {
+				(void) fflush(stdout);
 				(void) fsleep(interval);
 				continue;
 			}
@@ -5980,18 +5981,13 @@ zpool_do_iostat(int argc, char **argv)
 
 		}
 
-		/*
-		 * Flush the output so that redirection to a file isn't buffered
-		 * indefinitely.
-		 */
-		(void) fflush(stdout);
-
 		if (interval == 0)
 			break;
 
 		if (count != 0 && --count == 0)
 			break;
 
+		(void) fflush(stdout);
 		(void) fsleep(interval);
 	}
 
@@ -6514,6 +6510,8 @@ zpool_do_list(int argc, char **argv)
 			break;
 
 		pool_list_free(list);
+
+		(void) fflush(stdout);
 		(void) fsleep(interval);
 	}
 
@@ -9094,6 +9092,7 @@ zpool_do_status(int argc, char **argv)
 		if (count != 0 && --count == 0)
 			break;
 
+		(void) fflush(stdout);
 		(void) fsleep(interval);
 	}