btrfs: make should_throttle loop local in btrfs_truncate_inode_items

We reset this bool on every loop through the truncate loop, make this
variable local to the loop.

Reviewed-by: Filipe Manana <fdmanana@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>