Btrfs: add some free space cache tests

We keep hitting bugs in the tree log replay because btrfs_remove_free_space
doesn't account for some corner case.  So add a bunch of tests to try and fully
test btrfs_remove_free_space since the only time it is called is during tree log
replay.  These tests all finish successfully, so as we find more of these bugs
we need to add to these tests to make sure we don't regress in fixing things.
I've hidden the tests behind a Kconfig option, but they take no time to run so
all btrfs developers should have this turned on all the time.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>

fs/btrfs/free-space-cache.h

@@ -110,4 +110,9 @@ int btrfs_return_cluster_to_free_space(
 			       struct btrfs_free_cluster *cluster);
 int btrfs_trim_block_group(struct btrfs_block_group_cache *block_group,
 			   u64 *trimmed, u64 start, u64 end, u64 minlen);
+
+#ifdef CONFIG_BTRFS_FS_RUN_SANITY_TESTS
+void btrfs_test_free_space_cache(void);
+#endif
+
 #endif