dm btree: increase rebalance threshold in __rebalance2()

We got the following warnings from thin_check during thin-pool setup:

$ thin_check /dev/vdb
examining superblock
examining devices tree
missing devices: [1, 84]
too few entries in btree_node: 41, expected at least 42 (block 138, max_entries = 126)
examining mapping tree

The phenomenon is the number of entries in one node of details_info tree is
less than (max_entries / 3). And it can be easily reproduced by the following
procedures:

$ new a thin pool
$ presume the max entries of details_info tree is 126
$ new 127 thin devices (e.g. 1~127) to make the root node being full
and then split
$ remove the first 43 (e.g. 1~43) thin devices to make the children
reblance repeatedly
$ stop the thin pool
$ thin_check

The root cause is that the B-tree removal procedure in __rebalance2()
doesn't guarantee the invariance: the minimal number of entries in
non-root node should be >= (max_entries / 3).

Simply fix the problem by increasing the rebalance threshold to
make sure the number of entries in each child will be greater
than or equal to (max_entries / 3 + 1), so no matter which
child is used for removal, the number will still be valid.

Cc: [e-mail]
Signed-off-by: Hou Tao <[e-mail]>
Acked-by: Joe Thornber <[e-mail]>
Signed-off-by: Mike Snitzer <[e-mail]>

drivers/md/persistent-data/dm-btree-remove.c

···
 	struct btree_node *right = r->n;
 	uint32_t nr_left = le32_to_cpu(left->header.nr_entries);
 	uint32_t nr_right = le32_to_cpu(right->header.nr_entries);
-	unsigned threshold = 2 * merge_threshold(left) + 1;
+	/*
+	 * Ensure the number of entries in each child will be greater
+	 * than or equal to (max_entries / 3 + 1), so no matter which
+	 * child is used for removal, the number will still be not
+	 * less than (max_entries / 3).
+	 */
+	unsigned int threshold = 2 * (merge_threshold(left) + 1);
 
 	if (nr_left + nr_right < threshold) {
 		/*