Reinstate raw receive check when truncating

This patch re-adds a check that was removed in 369aa50. The check
confirms that a raw receive is not occuring before truncating an
object's dn_maxblkid. At the time, it was believed that all cases
that would hit this code path would be handled in other places,
but that was not the case.

Reviewed-by: Matt Ahrens <mahrens@delphix.com>
Reviewed-by: Paul Dagnelie <pcd@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Closes #8852 
Closes #8857

module/zfs/dnode_sync.c

@@ -384,7 +384,21 @@ dnode_sync_free_range_impl(dnode_t *dn, uint64_t blkid, uint64_t nblks,
 		}
 	}
 
-	if (trunc) {
+	/*
+	 * Do not truncate the maxblkid if we are performing a raw
+	 * receive. The raw receive sets the maxblkid manually and
+	 * must not be overridden. Usually, the last DRR_FREE record
+	 * will be at the maxblkid, because the source system sets
+	 * the maxblkid when truncating. However, if the last block
+	 * was freed by overwriting with zeros and being compressed
+	 * away to a hole, the source system will generate a DRR_FREE
+	 * record while leaving the maxblkid after the end of that
+	 * record. In this case we need to leave the maxblkid as
+	 * indicated in the DRR_OBJECT record, so that it matches the
+	 * source system, ensuring that the cryptographic hashes will
+	 * match.
+	 */
+	if (trunc && !dn->dn_objset->os_raw_receive) {
 		ASSERTV(uint64_t off);
 		dn->dn_phys->dn_maxblkid = blkid == 0 ? 0 : blkid - 1;