btrfs: cleanup main loop in btrfs_encoded_read_regular_fill_pages

btrfs_encoded_read_regular_fill_pages has a pretty odd control flow.
Unwind it so that there is a single loop over the pages array.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David Sterba <dsterba@suse.com>