BRT: Skip duplicate BRT prefetches

If there is a pending entry for this block, then we've already
issued BRT prefetch for it within this TXG, so don't do it again.
BRT vdev lookup and following zap_prefetch_uint64() call can be
pretty expensive and should be avoided when not necessary.

Reviewed-by: Pawel Jakub Dawidek <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by:	Alexander Motin <[email protected]>
Sponsored by:	iXsystems, Inc.
Closes #15941

module/zfs/brt.c

@@ -1472,10 +1472,10 @@ brt_pending_add(spa_t *spa, const blkptr_t *bp, dmu_tx_t *tx)
 		kmem_cache_free(brt_pending_entry_cache, newbpe);
 	} else {
 		ASSERT(bpe == NULL);
-	}
 
-	/* Prefetch BRT entry, as we will need it in the syncing context. */
-	brt_prefetch(brt, bp);
+		/* Prefetch BRT entry for the syncing context. */
+		brt_prefetch(brt, bp);
+	}
 }
 
 void