crimson/os/seastore: validate node immediately prior to adding pin

crimson/os/seastore: validate node immediately prior to adding pin

Mostly, operating on an invalid extent is harmless by construction --
it'll be caught by the read set check in submit_transaction.  However,
prior to adding an extent to the lba pin set, we need to check that it's
really still live, which we can do by checking that it and its immediate
parent are still valid.  Mutating the target extent would have
invalidated it, and replacing the target extent would necessarily have
involved a mutation to the parent, which would have invalidated it.

Signed-off-by: Samuel Just <sjust@redhat.com>