Don't hard-code number of ioctls for portability

Use (ZFS_IOC_LAST - ZFS_IOC_FIRST) instead of 256.
It seems 256 is just a number large enough to hold ioctls
at the moment.

Using 256 also causes compile-time warning or error
on platfoms whose enum zfs_ioc definition differs.

Reviewed-by: Olaf Faaland <[email protected]>
Reviewed by: Brian Behlendorf <[email protected]>
Reviewed-by: Igor Kozhukhov <[email protected]>
Signed-off-by: Tomohiro Kusumi <[email protected]>
Closes openzfs#8598

tests/zfs-tests/cmd/libzfs_input_check/libzfs_input_check.c

@@ -37,7 +37,7 @@ const char *active_test;
 /*
  * Tracks which zfs_ioc_t commands were tested
  */
-boolean_t ioc_tested[256];
+boolean_t ioc_tested[ZFS_IOC_LAST - ZFS_IOC_FIRST];
 
 /*
  * Legacy ioctls that are skipped (for now)