zdb: adjust block histogram binning strategy

Previously, a bin included all blocks _starting_ from given size
(e.g., a "4K" bin would include all blocks within the [4K; 8K) region).
This is counter-intuitive and does not match the typical use-case of the
block histogram (that is, to estimate disk usage considering how ZFS'
block allocation works). In other words, if I'm looking at the "4K" row,
I'm interested in records that _fit into_ a 4K block.

Adjust the binning strategy such that a bin includes all blocks _up to_
given size, such that e.g. a "4K" bin would include all blocks within
the (2K; 4K] region.

Signed-off-by: Ivan Shapovalov <[email protected]>

cmd/zdb/zdb.c

@@ -6139,7 +6139,12 @@ zdb_count_block(zdb_cb_t *zcb, zilog_t *zilog, const blkptr_t *bp,
 	 */
 	int bin;
 
-#define BIN(size) (highbit64(size) - 1)
+	/*
+	 * Binning strategy: each bin includes blocks up to and including
+	 * the given size (excluding blocks that fit into the previous bin).
+	 * This way, the "4K" bin includes blocks within the (2K; 4K] range.
+	 */
+#define BIN(size) (highbit64((size) - 1))
 
 	switch (block_bin_mode) {
 	case BIN_PSIZE: bin = BIN(BP_GET_PSIZE(bp)); break;