Fix long_free_dirty accounting for small files

For files smaller than recordsize, it's most likely that they don't have
L1 blocks. However, current calculation will always return at least 1 L1
block.

In this change, we check dnode level to figure out if it has L1 blocks
or not, and return 0 if it doesn't. This will reduce the chance of
unnecessary throttling when deleting a large number of small files.

Signed-off-by: Chunwei Chen <[email protected]>

module/zfs/dmu.c

@@ -827,6 +827,9 @@ get_next_chunk(dnode_t *dn, uint64_t *start, uint64_t minimum, uint64_t *l1blks)
 	if (total_l1blks <= maxblks) {
 		*l1blks = total_l1blks;
 		*start = minimum;
+		/* Don't count when we don't have L1 blocks */
+		if (dn->dn_nlevels <= 1)
+			*l1blks = 0;
 		return (0);
 	}
 	ASSERT(ISP2(iblkrange));