bcachefs: add btree_node_merging_disabled debug param

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/bcachefs.h

@@ -359,6 +359,8 @@ do {									\
 #define BCH_DEBUG_PARAMS_ALWAYS()					\
 	BCH_DEBUG_PARAM(key_merging_disabled,				\
 		"Disables merging of extents")				\
+	BCH_DEBUG_PARAM(btree_node_merging_disabled,			\
+		"Disables merging of btree nodes")			\
 	BCH_DEBUG_PARAM(btree_gc_always_rewrite,			\
 		"Causes mark and sweep to compact and rewrite every "	\
 		"btree node it traverses")				\

fs/bcachefs/btree_update_interior.h

@@ -144,6 +144,9 @@ static inline int bch2_foreground_maybe_merge_sibling(struct btree_trans *trans,
 
 	EBUG_ON(!btree_node_locked(path, level));
 
+	if (bch2_btree_node_merging_disabled)
+		return 0;
+
 	b = path->l[level].b;
 	if (b->sib_u64s[sib] > trans->c->btree_foreground_merge_threshold)
 		return 0;