pnfs/blocklayout: move extent processing to blocklayout.c

This isn't device(id) related, so move it into the main file.  Simple move
for now, the next commit will clean it up a bit.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>