Revert "OpenZFS 9036 - zfs: duplicate 'const' declaration specifier"

This reverts commit cbb893321545c2c9052787b556c9375fcb103979.

The original change in OpenZFS 9036 did remove duplicate 'const'
specifiers, but the ZoL port had already done what *should* have been
done in OpenZFS 9036, which is to make the pointers themselves const.
The port of the change to ZoL ended up doing an unnecessary removal
of the constness of the pointers. Undo that.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Ari Sundholm <ari@tuxera.com>
Closes #7444