The allocator usually doesn't increment bucket gens right away on
buckets that it's about to hand out (for reasons that need to be
documented), instead deferring that to whatever extent update first
references that bucket.

But stripe pointers reference buckets without changing bucket sector
counts, meaning we could end up with a pointer in a stripe with a gen
newer than the bucket it points to.

Fix this by adding a transactional trigger for KEY_TYPE_stripe that just
writes out the keys in the alloc btree for the buckets it points to.

Also - consolidate the code that checks pointer validity.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>