btrfs: let the assertion expression compile in all configs

A compiler warning (in a patch in development) pointed to a variable
that was used only inside and ASSERT:

  u64 root_objectid = root->root_key.objectid;
  ASSERT(root_objectid == ...);

  fs/btrfs/relocation.c: In function ‘insert_dirty_subv’:
  fs/btrfs/relocation.c:2138:6: warning: unused variable ‘root_objectid’ [-Wunused-variable]
    u64 root_objectid = root->root_key.objectid;
	^~~~~~~~~~~~~

When CONFIG_BRTFS_ASSERT isn't enabled, variable root_objectid isn't used.

Rework the assertion helper by adding a runtime check instead of the
'#ifdef CONFIG_BTRFS_ASSERT #else ...", so the compiler sees the
condition being passed into an inline function after preprocessing.
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Reviewed-by: David Sterba <dsterba@suse.com>
[ update changelog ]
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.h

@@ -3518,21 +3518,18 @@ do {								\
 	rcu_read_unlock();					\
 } while (0)
 
-#ifdef CONFIG_BTRFS_ASSERT
-
 __cold
 static inline void assfail(const char *expr, const char *file, int line)
 {
+	if (IS_ENABLED(CONFIG_BTRFS_ASSERT)) {
 		pr_err("assertion failed: %s, file: %s, line: %d\n",
 		       expr, file, line);
 		BUG();
+	}
 }
 
 #define ASSERT(expr)	\
 	(likely(expr) ? (void)0 : assfail(#expr, __FILE__, __LINE__))
-#else
-#define ASSERT(expr)	((void)0)
-#endif
 
 /*
  * Use that for functions that are conditionally exported for sanity tests but