Fix the zfs_ioctl compat layer to support zfs_cmd size change introduced
in r247265 (ZFS deadman thread). Both new utilities now support the old
kernel and new kernel properly detects old utilities.

For future backwards compatibility, the vfs.zfs.version.ioctl read-only
sysctl has been introduced. With this sysctl zfs utilities will be able
to detect the ioctl interface version of the currently loaded zfs module.

As a side effect, the zfs utilities between r247265 and this revision don't
support the old kernel module. If you are using HEAD newer or equal than
r247265, install the new kernel module (or whole kernel) first.

MFC after:	10 days