btrfs: drop min_size from evict_refill_and_join

We don't need it, rsv->size is set once and never changes throughout
its lifetime, so just use that for the reserve size.

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