mon: OSDMonitor: allow adding tiers to FS pools

This was an overly-strict check.  In fact it is perfectly
fine to set an overlay on a pool that is already in use
as a filesystem data or metadata pool.

Fixes: #10135

Signed-off-by: John Spray <john.spray@redhat.com>
Merge pull request #3008 from ceph/wip-10135

mon: OSDMonitor: allow adding tiers to FS pools

Reviewed-by: Greg Farnum <gfarnum@redhat.com>