PG::calc_ec_acting: use MIN(auth_info.log_tail, info.log_tail) for backfill

Otherwise, if we have a longer tail, we might deem a particular osd to
be backfill in calc_ec_acting, but not in activate().

Fixes: #7636
Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge branch 'wip-7696' into wip-sam-testing