btrfs: Open code insert_extent_backref

No need to add a level of indirection for hiding a simple 'if'. Open
code insert_extent_backref in its sole caller. No functional changes.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>