mds: avoid spurious TMAP2OMAP warning

The message "one or more OSDs do not support TMAP2OMAP" was printed
incorrectly when zero OSDs were up (and therefore the feature was
absent).  Don't issue this prompt until at least one OSD is up.

Signed-off-by: John Spray <john.spray@inktank.com>
Merge pull request #1481 from ceph/wip-tmap2omap-warn

mds: avoid spurious TMAP2OMAP warning

Reviewed-by: Sage Weil <sage@inktank.com>