btree: avoid variable-length allocations

geo->keylen cannot be larger than 4.  So we might as well make
fixed-size allocations.

Given the one remaining user, geo->keylen cannot even be larger than 1.
Logfs used to have 64bit and 128bit keys, tcm_qla2xxx only has 32bit
keys.  But let's not break the code if we don't have to.
Signed-off-by: Joern Engel <joern@purestorage.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

lib/btree.c

@@ -3,7 +3,7 @@
  *
  * As should be obvious for Linux kernel code, license is GPLv2
  *
- * Copyright (c) 2007-2008 Joern Engel <joern@logfs.org>
+ * Copyright (c) 2007-2008 Joern Engel <joern@purestorage.com>
  * Bits and pieces stolen from Peter Zijlstra's code, which is
  * Copyright 2007, Red Hat Inc. Peter Zijlstra
  * GPLv2
@@ -76,6 +76,8 @@ struct btree_geo btree_geo128 = {
 };
 EXPORT_SYMBOL_GPL(btree_geo128);
 
+#define MAX_KEYLEN	(2 * LONG_PER_U64)
+
 static struct kmem_cache *btree_cachep;
 
 void *btree_alloc(gfp_t gfp_mask, void *pool_data)
@@ -313,7 +315,7 @@ void *btree_get_prev(struct btree_head *head, struct btree_geo *geo,
 {
 	int i, height;
 	unsigned long *node, *oldnode;
-	unsigned long *retry_key = NULL, key[geo->keylen];
+	unsigned long *retry_key = NULL, key[MAX_KEYLEN];
 
 	if (keyzero(geo, __key))
 		return NULL;
@@ -639,8 +641,8 @@ EXPORT_SYMBOL_GPL(btree_remove);
 int btree_merge(struct btree_head *target, struct btree_head *victim,
 		struct btree_geo *geo, gfp_t gfp)
 {
-	unsigned long key[geo->keylen];
-	unsigned long dup[geo->keylen];
+	unsigned long key[MAX_KEYLEN];
+	unsigned long dup[MAX_KEYLEN];
 	void *val;
 	int err;