xfs: reflink with large realtime extents

Now that we've landed support for reflink on the realtime device for
cases where the rt extent size is the same as the fs block size, enhance
the reflink code further to support cases where the rt extent size is a
power-of-two multiple of the fs block size.  This enables us to do data
block sharing (for example) for much larger allocation units by dirtying
pagecache around shared extents and expanding writeback to write back
shared extents fully.