Disable spill block processing.

This patch might allow some pools with corrupted dnodes to be imported and
for their filesystems to be destroyed.  All spill blocks are ignored by
virtue of defining DNODE_FLAG_SPILL_BLKPTR as 0.  The only other change
is to remove a number of assertions which are typically triggered when
removing filesystems.

WARNING: VERY DANGEROUS!  I've given it a single round of testing
consisting of:

  - Given an existing filesystem with xattr=sa and a file(s) with long
    xattrs needing spill blocks.
  - Destroy the filesystem with this patch.  - Run zdb to verify the
    leaked space.

include/sys/dnode.h

@@ -117,7 +117,7 @@ enum dnode_dirtycontext {
 #define	DNODE_FLAG_USERUSED_ACCOUNTED	(1<<1)
 
 /* Does dnode have a SA spill blkptr in bonus? */
-#define	DNODE_FLAG_SPILL_BLKPTR	(1<<2)
+#define	DNODE_FLAG_SPILL_BLKPTR	0
 
 typedef struct dnode_phys {
 	uint8_t dn_type;		/* dmu_object_type_t */

module/zfs/bptree.c

@@ -94,9 +94,11 @@ bptree_free(objset_t *os, uint64_t obj, dmu_tx_t *tx)
 	VERIFY3U(0, ==, dmu_bonus_hold(os, obj, FTAG, &db));
 	bt = db->db_data;
 	ASSERT3U(bt->bt_begin, ==, bt->bt_end);
+#if 0
 	ASSERT0(bt->bt_bytes);
 	ASSERT0(bt->bt_comp);
 	ASSERT0(bt->bt_uncomp);
+#endif
 	dmu_buf_rele(db, FTAG);
 
 	return (dmu_object_free(os, obj, tx));
@@ -290,9 +292,11 @@ bptree_iterate(objset_t *os, uint64_t obj, boolean_t free, bptree_itor_t func,
 			ba.ba_phys->bt_uncomp = 0;
 		}
 
+#if 0
 		ASSERT0(ba.ba_phys->bt_bytes);
 		ASSERT0(ba.ba_phys->bt_comp);
 		ASSERT0(ba.ba_phys->bt_uncomp);
+#endif
 	}
 
 	dmu_buf_rele(db, FTAG);

module/zfs/dsl_destroy.c

@@ -671,8 +671,10 @@ dsl_dir_destroy_sync(uint64_t ddobj, dmu_tx_t *tx)
 	 */
 	dsl_dir_set_reservation_sync_impl(dd, 0, tx);
 
+#if 0
 	ASSERT0(dd->dd_phys->dd_used_bytes);
 	ASSERT0(dd->dd_phys->dd_reserved);
+#endif
 	for (t = 0; t < DD_USED_NUM; t++)
 		ASSERT0(dd->dd_phys->dd_used_breakdown[t]);
 

module/zfs/dsl_scan.c

@@ -1559,12 +1559,14 @@ dsl_scan_sync(dsl_pool_t *dp, dmu_tx_t *tx)
 		    -dp->dp_free_dir->dd_phys->dd_compressed_bytes,
 		    -dp->dp_free_dir->dd_phys->dd_uncompressed_bytes, tx);
 	}
+#if 0
 	if (!scn->scn_async_destroying) {
 		/* finished; verify that space accounting went to zero */
 		ASSERT0(dp->dp_free_dir->dd_phys->dd_used_bytes);
 		ASSERT0(dp->dp_free_dir->dd_phys->dd_compressed_bytes);
 		ASSERT0(dp->dp_free_dir->dd_phys->dd_uncompressed_bytes);
 	}
+#endif
 
 	if (scn->scn_phys.scn_state != DSS_SCANNING)
 		return;