We want to limit the scope of the extent lock to be around operations
that can change in flight.  Currently we hold the extent lock through
the entire writepage operation, which isn't really necessary.

We want to protect to make sure nobody has updated DELALLOC.  In
find_lock_delalloc_range we must lock the range in order to validate the
contents of our io_tree.  However once we've done that we're safe to
unlock the range and continue, as we have the page lock already held for
the range.

We are protected from all operations at this point.

* mmap() - we're holding the page lock, thus are protected.
* buffered writes - again, we're protected because we take the page lock
  for the first and last page in our range for buffered writes so we
  won't create new delalloc ranges in this area.
* direct IO - we invalidate pagecache before attempting to write a new
  area, which requires the page lock, so again are protected once we're
  holding the page lock on this range.

Additionally this behavior actually already exists for compressed, we
unlock the range as soon as we start to process the async extents, and
re-lock it during compression.  So this is completely safe, and makes
the locking more consistent.

Make this simple by just pushing the extent lock into
btrfs_run_delalloc_range.  From there followup patches will push the
lock further down into its users.
Reviewed-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>