Btrfs: fix compiles when CONFIG_BTRFS_FS_RUN_SANITY_TESTS is off

Commit fccb84c94 moved added some helpers to cleanup our sanity tests,
but it looks like both Dave and I always compile with the tests enabled.

This fixes things to work when they are turned off too.

Signed-off-by: Chris Mason <clm@fb.com>
diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index b94c1c7..d557264e 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -1818,9 +1818,8 @@
 
 	u64 highest_objectid;
 
-#ifdef CONFIG_BTRFS_FS_RUN_SANITY_TESTS
+	/* only used with CONFIG_BTRFS_FS_RUN_SANITY_TESTS is enabled */
 	u64 alloc_bytenr;
-#endif
 
 	u64 defrag_trans_start;
 	struct btrfs_key defrag_progress;