A new helper, defrag_one_range(), is introduced to defrag one range.

This function will mostly prepare the needed pages and extent status for
defrag_one_locked_target().

As we can only have a consistent view of extent map with page and extent
bits locked, we need to re-check the range passed in to get a real
target list for defrag_one_locked_target().

Since defrag_collect_targets() will call defrag_lookup_extent() and lock
extent range, we also need to teach those two functions to skip extent
lock.  Thus new parameter, @locked, is introduced to skip extent lock if
the caller has already locked the range.
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>