bcachefs: Inline bch2_trans_kmalloc() fast path

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

fs/bcachefs/btree_iter.c

@@ -2668,17 +2668,15 @@ void bch2_trans_copy_iter(struct btree_iter *dst, struct btree_iter *src)
 	dst->key_cache_path = NULL;
 }
 
-void *bch2_trans_kmalloc(struct btree_trans *trans, size_t size)
+void *__bch2_trans_kmalloc(struct btree_trans *trans, size_t size)
 {
 	unsigned new_top = trans->mem_top + size;
-	void *p;
-
-	trans->mem_max = max(trans->mem_max, new_top);
-
-	if (new_top > trans->mem_bytes) {
 	size_t old_bytes = trans->mem_bytes;
 	size_t new_bytes = roundup_pow_of_two(new_top);
 	void *new_mem;
+	void *p;
+
+	trans->mem_max = max(trans->mem_max, new_top);
 
 	WARN_ON_ONCE(new_bytes > BTREE_TRANS_MEM_MAX);
 
@@ -2699,7 +2697,6 @@ void *bch2_trans_kmalloc(struct btree_trans *trans, size_t size)
 		trace_and_count(trans->c, trans_restart_mem_realloced, trans, _RET_IP_, new_bytes);
 		return ERR_PTR(btree_trans_restart(trans, BCH_ERR_transaction_restart_mem_realloced));
 	}
-	}
 
 	p = trans->mem + trans->mem_top;
 	trans->mem_top += size;

fs/bcachefs/btree_iter.h

@@ -328,7 +328,23 @@ static inline void set_btree_iter_dontneed(struct btree_iter *iter)
 		iter->path->preserve = false;
 }
 
-void *bch2_trans_kmalloc(struct btree_trans *, size_t);
+void *__bch2_trans_kmalloc(struct btree_trans *, size_t);
+
+static inline void *bch2_trans_kmalloc(struct btree_trans *trans, size_t size)
+{
+	unsigned new_top = trans->mem_top + size;
+	void *p = trans->mem + trans->mem_top;
+
+	if (likely(new_top <= trans->mem_bytes)) {
+		trans->mem_top += size;
+		memset(p, 0, size);
+		return p;
+	} else {
+		return __bch2_trans_kmalloc(trans, size);
+
+	}
+}
+
 u32 bch2_trans_begin(struct btree_trans *);
 
 static inline struct btree *