bcachefs: Redo checks for sufficient devices

bcachefs: Redo checks for sufficient devices

When the replicas mechanism was added, for tracking data by which drives
it's replicated on, the check for whether we have sufficient devices was
never updated to make use of it. This patch finally does that.

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