crimson/os/seastore/lba_manager/btree: fix get_val() paddr value from iterator
This was causing a stray RETIRED_PLACEHOLDER to be created resulting in
a segment cleaner segfault in release on an invalid segment and a crash
upon adding a duplicate lba btree pin since the returned addr didn't
match the addr of the INITIAL_PENDING extent on the transaction.
Fixes: https://tracker.ceph.com/issues/52434 Fixes: https://tracker.ceph.com/issues/52435 Signed-off-by: Samuel Just <sjust@redhat.com>