submit_extent_page always returns 0 since commit d5e4377d ("btrfs:
split zone append bios in btrfs_submit_bio").  Change it to a void return
type and remove all the unreachable error handling code in the callers.
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>