btrfs: cleanup calculation of lockend in lock_and_cleanup_extent_if_need()

We're just doing rounding up to sectorsize to calculate the lockend.
There is no need to do the unnecessary length calculation, just direct
round_up() is enough.

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