Add a runtime feature (-R) flag for the free space tree. A filesystem
that is mkfs'd with -R free-space-tree then mounted with no options has
the same contents as one mkfs'd without the option, then mounted with
'-o space_cache=v2'.
The only tricky thing is in exactly how to call the tree creation code.
Using btrfs_create_free_space_tree as is did not quite work, because an
extra reference to the eb (root->commit_root) is leaked, which mkfs
complains about with a warning. I opted to follow how the uuid tree is
created by adding it to the dirty roots list for cleanup by
commit_tree_roots in commit_transaction. As a result,
btrfs_create_free_space_tree no longer exactly matches the version in
the kernel sources.
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Boris Burkov <boris@bur.io>
Signed-off-by: David Sterba <dsterba@suse.com>
