Check b_freeze_cksum under ZFS_DEBUG_MODIFY conditional

Check b_freeze_cksum under ZFS_DEBUG_MODIFY conditional

The b_freeze_cksum field can only have data when ZFS_DEBUG_MODIFY
is set.  Therefore, the EQUIV check must be wrapped accordingly.
For the same reason the ASSERT in arc_buf_fill() in unsafe.
However, since it's largely redundant it has simply been removed.

Reviewed-by: George Wilson <gwilson@delphix.com>
Reviewed-by: Allan Jude <allanjude@freebsd.org>
Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #8979