btrfs: defrag: factor out page preparation into a helper

In cluster_pages_for_defrag(), we have complex code block inside one
for() loop.

The code block is to prepare one page for defrag, this will ensure:

- The page is locked and set up properly.
- No ordered extent exists in the page range.
- The page is uptodate.

This behavior is pretty common and will be reused by later defrag
rework.

So factor out the code into its own helper, defrag_prepare_one_page(),
for later usage, and cleanup the code by a little.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>