bcachefs: Check for bad needs_discard before doing discard

In the discard worker, we were failing to validate the bucket state -
meaning a corrupt needs_discard btree could cause us to discard a bucket
that we shouldn't.

If check_alloc_info hasn't run yet we just want to bail out, otherwise
it's a filesystem inconsistent error.

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