crimson: fix mismatch between backfill's enqueue_push() and recovery.

crimson: fix mismatch between backfill's enqueue_push() and recovery.

The unittesting discovered an issue between `BackfillState` and
the low-layer recovery code: `RecoveryBackend::recover_object()`.
`BackfillState::enqueue_push()` was assuming it controlls also
to which backfill target the push is sent while `recover_object()`
calculated the set on its own.
Fixing this is the reason why `enqueue_push()` loses the `const
pg_shard_t& target` parameter.

Signed-off-by: Radoslaw Zarzynski <rzarzyns@redhat.com>