bcachefs: Fix missing alloc_data_type_set()

Incorrect bucket state transition in the discard path; when incrementing
a bucket's generation number that had already been discarded, we were
forgetting to check if it should be need_gc_gens, not free.

This was caught by the .invalid checks in the transaction commit path,
causing us to go emergency read only.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>