btrfs: factor out stripe length calculation into a helper

Currently there are two locations which need to calculate the real
length of a stripe (which can be at the end of a chunk, and the chunk
size may not always be 64K aligned).

Factor them into a helper as we're going to have a third user soon.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>