mds: fix asok on rank 0

Broken by:
1b2b34431 (MDS: do not allow invocation of most commands on an inactive MDS)

MDS::whoami is a rank, so check for inactive is <0, not <=0

Signed-off-by: John Spray <john.spray@redhat.com>
Merge pull request #3202 from ceph/wip-fix-asok

mds: fix asok on rank 0