mds: rename 'mix stale' => 'mix_stale'
mds: rename 'mix stale' => 'mix_stale'

For unambigous debug output

Signed-off-by: Sage Weil <6dd34506bbd5e58221bbb3e4732d97c91f02277b@newdream.net>

ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph