dsa: Combine core and tagging code

These files have circular dependencies, so if we make DSA modular then
they must be built into the same module.  Therefore, link them
together and merge their respective module init and exit functions.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>