Fix deduplication of overridden blocks

Implementation of DDT pruning introduced verification of DVAs in
a block pointer during ddt_lookup() to not by mistake free previous
pruned incarnation of the entry.  But when writing a new block in
zio_ddt_write() we might have the DVAs only from override pointer,
which may never have "D" flag to be confused with pruned DDT entry,
and we'll abandon those DVAs if we find a matching entry in DDT.

This fixes deduplication for blocks written via dmu_sync() for
purposes of indirect ZIL write records, that I have tested.  And
I suspect it might actually allow deduplication for Direct I/O,
even though in an odd way -- first write block directly and then
delete it later during TXG commit if found duplicate, which part
I haven't tested.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.
Closes #17120

cmd/zdb/zdb.c

@@ -5864,7 +5864,7 @@ zdb_count_block(zdb_cb_t *zcb, zilog_t *zilog, const blkptr_t *bp,
 		 * Find the block. This will create the entry in memory, but
 		 * we'll know if that happened by its refcount.
 		 */
-		ddt_entry_t *dde = ddt_lookup(ddt, bp);
+		ddt_entry_t *dde = ddt_lookup(ddt, bp, B_TRUE);
 
 		/*
 		 * ddt_lookup() can return NULL if this block didn't exist