btrfs: remove unlock_extent from run_delalloc_compressed

Since we immediately unlock the extent range when we enter
run_delalloc_compressed() simply move the lock_extent() down to cover
cow_file_range() and then remove the unlock_extent() from
run_delalloc_compressed.

Reviewed-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>