drbd: split off drbd_buildtag into separate file

To be more similar to what we do in the out-of-tree module and ease the
upstreaming process.

Signed-off-by: Christoph Böhmwalder <christoph.boehmwalder@linbit.com>
Reviewed-by: Joel Colledge <joel.colledge@linbit.com>
Link: https://lore.kernel.org/r/20230113123506.144082-2-christoph.boehmwalder@linbit.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>