btrfs: do not try to unlock the extent for non-subpage metadata reads

Only subpage metadata reads lock the extent.  Don't try to unlock it and
waste cycles in the extent tree lookup for PAGE_SIZE or larger metadata.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David Sterba <dsterba@suse.com>