btrfs-progs: Add support for tree block operations on fs_info without roots

btrfs-progs: Add support for tree block operations on fs_info without roots

Since open_ctree_fs_info() now may return a fs_info even without any
roots, modify functions like read_tree_block() to operate with such
fs_info.

This provides the basis for btrfs-find-root to operate on chunk tree
with corrupted fs.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
[ coding style adjustments, unified declarations ]
Signed-off-by: David Sterba <dsterba@suse.com>