btrfs: make relocation use btrfs_read_tree_root()

Relocation has it's special roots, we don't want to save these in the
root cache either, so swap it to use btrfs_read_tree_root().  However
the reloc root does need REF_COWS set, so make sure we set it everywhere
we use this helper, as it no longer does the REF_COWS setting.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>