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/extent_io.h b/fs/btrfs/extent_io.h
index f40d67f..6d4b938 100644
--- a/fs/btrfs/extent_io.h
+++ b/fs/btrfs/extent_io.h
@@ -374,7 +374,7 @@
 				      struct extent_io_tree *tree,
 				      struct page *locked_page, u64 *start,
 				      u64 *end, u64 max_bytes);
+#endif
 struct extent_buffer *alloc_test_extent_buffer(struct btrfs_fs_info *fs_info,
 					       u64 start, unsigned long len);
 #endif
-#endif