btrfs: avoid unnecessary reclaim calculation in priority_reclaim_metadata_space()

btrfs: avoid unnecessary reclaim calculation in priority_reclaim_metadata_space()

If the given ticket was already served (its ->bytes is 0), then we wasted
time calculating the metadata reclaim size. So calculate it only after we
checked the ticket was not yet served.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>