btrfs: extend btrfs_cleanup_ordered_extents for NULL locked_page

btrfs_cleanup_ordered_extents() assumes locked_page to be non-NULL, so it
is not usable for submit_uncompressed_range() which can have NULL
locked_page.

Add support supports locked_page == NULL case. Also, it rewrites
redundant "page_offset(locked_page)".

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>