btrfs: remove stripe boundary calculation for encoded I/O

Stop looking at the stripe boundary in
btrfs_encoded_read_regular_fill_pages() now that btrfs_submit_bio can
split bios.

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