afs: Automatically generate trace tag enums

Automatically generate trace tag enums from the symbol -> string mapping
tables rather than having the enums as well, thereby reducing duplicated
data.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: linux-afs@lists.infradead.org
cc: linux-fsdevel@vger.kernel.org