fs: btrfs: Crossport read_tree_block() from btrfs-progs

This is the one of the basic stone function for btrfs, which:
- Resolves the chunk mappings
- Reads data from disk
- Does various sanity check

With read_tree_block(), we can finally crossport needed btrfs btree
operations to U-Boot.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Marek Behún <marek.behun@nic.cz>