DLPX-75470 estat zpl and arc_prefetch scripts need znode parameter

[brad-lewis]

The estat zpl and arc_prefetch scripts do not catch any data due to the inode parameter in zfs_read and zfs_write being replaced by a znode.

With this change the scripts start producing output:

04/23/21 - 21:13:28 UTC

 Tracing enabled... Hit Ctrl-C to end.
   microseconds                                          zfs_write async
value range                 count ------------- Distribution ------------- 
[20, 30)                        5 |@@@                                     
[30, 40)                        3 |@@                                      
[40, 50)                       27 |@@@@@@@@@@@@@@@@@                       
[50, 60)                       18 |@@@@@@@@@@@                             
[60, 70)                        3 |@@                                      
[70, 80)                        2 |@                                       
[100, 200)                      4 |@@                                      

                                       iops(/s)  avg latency(us)       stddev(us)  throughput(k/s)
zfs_write async                              15               55              972              132


                                       iops(/s)  throughput(k/s)
total                                        15              132

04/23/21 - 21:14:20 UTC

prefetch hit count   : 1