bcachefs: new_stripe_alloc_buckets() takes alloc_request

bcachefs: new_stripe_alloc_buckets() takes alloc_request

More stack usage improvements: instead of creating a new alloc_request
(currently on the stack), save/restore just the fields we need to reuse.

This is a bit tricky, because we're doing a normal alloc_foreground.c
allocation, which calls into ec.c to get a stripe, which then does more
normal allocations - some of the fields get reused, and used
differently.

So we have to save and restore them - but the stack usage improvements
will be well worth it.

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