btrfs-progs: Refactor read_tree_block to get rid of btrfs_root

btrfs-progs: Refactor read_tree_block to get rid of btrfs_root

The only reasom read_tree_block() needs a btrfs_root parameter is to get
its node/sector size.

And long ago, I have already introduced a compactible interface,
read_tree_block_fs_info() to pass btrfs_fs_info instead of btrfs_root.

Since we have cleaned up all root->sector/node/stripesize users, we
should be OK to refactor read_tree_block() function.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>