bcachefs: Fix stripe reuse path

bcachefs: Fix stripe reuse path

It's possible that we reuse a stripe that doesn't have quite the same
configuration as the stripe_head we're allocating from. In that case, we
have to make sure that the new stripe uses the settings from the stripe
we resue, not the stripe head, and make sure the buffer is allocated
correctly.

This fixes the ec_mixed_tiers test.

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