mon: fix potential divide by zero on can_mark_{down,out}

Signed-off-by: Sage Weil <sage@redhat.com>
mon: fix divide by zero when pg_num adjusted and no osds

Fixes: #9101
Backport: firefly, dumpling
Signed-off-by: Sage Weil <sage@redhat.com>
Merge pull request #2250 from ceph/wip-9101

mon: fix divide by zero when no osds

Reviewed-by: Samuel Just <sam.just@inktank.com>