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 openzfs#8979

module/zfs/arc.c

@@ -1872,7 +1872,8 @@ arc_buf_try_copy_decompressed_data(arc_buf_t *buf)
 	 * There were no decompressed bufs, so there should not be a
 	 * checksum on the hdr either.
 	 */
-	EQUIV(!copied, hdr->b_l1hdr.b_freeze_cksum == NULL);
+	if (zfs_flags & ZFS_DEBUG_MODIFY)
+		EQUIV(!copied, hdr->b_l1hdr.b_freeze_cksum == NULL);
 
 	return (copied);
 }
@@ -2253,7 +2254,6 @@ arc_buf_fill(arc_buf_t *buf, spa_t *spa, const zbookmark_phys_t *zb,
 		 */
 		if (arc_buf_try_copy_decompressed_data(buf)) {
 			/* Skip byteswapping and checksumming (already done) */
-			ASSERT3P(hdr->b_l1hdr.b_freeze_cksum, !=, NULL);
 			return (0);
 		} else {
 			error = zio_decompress_data(HDR_GET_COMPRESS(hdr),