Don't hard-code number of ioctls for portability

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 <faaland1@llnl.gov>
Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Closes #8598