bcachefs: Handle -BCH_ERR_need_mark_replicas in gc

Locking considerations (possibly no longer relevant?) mean that when an
accounting update needs a new superblock replicas entry to be created,
it's deferred to the transaction commit error path.

But accounting updates for gc/fcsk aren't done from the transaction
commit path - so we need to handle
-BCH_ERR_btree_insert_need_mark_replicas locally.

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