bcachefs: Refactor replicas code

bcachefs: Refactor replicas code

Awhile back the mechanism for garbage collecting unused replicas entries
was significantly improved, but some cleanup was missed - this patch
does that now.

This is also prep work for a patch to account for erasure coded parity
blocks separately - we need to consolidate the logic for
checking/marking the various replicas entries from one bkey into a
single function.

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