crimson/os/seastore/cache: init extents prior to read

crimson/os/seastore/cache: init extents prior to read

Thus should ensure that any captured members of extent_init_func are
still valid at the cost of not being able to access the contents of the
extent at invocation time.  With this, we should be able to rely on any
logical extents/lba extents in the cache having validly initialized lba
pins.

Fixes: https://tracker.ceph.com/issues/53555
Signed-off-by: Samuel Just <sjust@redhat.com>