ReplicatedPG::recover_backfill: adjust last_backfill to HEAD if snapdir

Otherwise, if last_backfill_started is a snapdir, we will fail to send a
transaction for a client IO creating the head object and removing the
snapdir object.  The result will be that head will eventually be
backfilled, but the snapdir object will erroneously not be removed.

Fixes: #6685
Signed-off-by: Samuel Just <sam.just@inktank.com>