zfs: no need to check that size of zfs_cmd_t is not greater than IOCPARM_MAX

Nowadays we do not pass zfs_cmd_t directly through the ioctl interface.
Instead a small zfs_iocparm_t object is passed and the command is
explicitly copied in and out. So, the check has become irrelevant.

MFC after: 3 weeks
Sponsored by: Panzura