Don't overrun the block budget in server_log_merge_free_work().

This fixes a potential fence post failure like the following:

error: 1 holders exceeded alloc budget av: bef 7407 now 7392, fr: bef 8185 now 7672

The code is only accounting for the freed btree blocks, not the dirtying of
other items. So it's possible to be at exactly (COMMIT_HOLD_ALLOC_BUDGET / 2),
dirty some log btree blocks, loop again, then consume another
(COMMIT_HOLD_ALLOC_BUDGET / 2) and blow past the total budget.

In this example, we went over by 13 blocks.

By only consuming up to 1/8 of the budget on each loop, and committing when we
have consumed 3/4 of the budget, we can avoid the fence post condition.

Signed-off-by: Chris Kirby <ckirby@versity.com>

kmod/src/server.c

@@ -2531,7 +2531,7 @@ static void server_log_merge_free_work(struct work_struct *work)
 
 		ret = scoutfs_btree_free_blocks(sb, &server->alloc,
 						&server->wri, &fr.key,
-						&fr.root, COMMIT_HOLD_ALLOC_BUDGET / 2);
+						&fr.root, COMMIT_HOLD_ALLOC_BUDGET / 8);
 		if (ret < 0) {
 			err_str = "freeing log btree";
 			break;
@@ -2550,7 +2550,7 @@ static void server_log_merge_free_work(struct work_struct *work)
 		/* freed blocks are in allocator, we *have* to update fr */
 		BUG_ON(ret < 0);
 
-		if (server_hold_alloc_used_since(sb, &hold) >= COMMIT_HOLD_ALLOC_BUDGET / 2) {
+		if (server_hold_alloc_used_since(sb, &hold) >= (COMMIT_HOLD_ALLOC_BUDGET * 3) / 4) {
 			mutex_unlock(&server->logs_mutex);
 			ret = server_apply_commit(sb, &hold, ret);
 			commit = false;