ceph: fix version check on racing inode updates

We may get updates on the same inode from multiple MDSs; generally we only
pay attention if the update is newer than what we already have.  The
exception is when an MDS sense unstable information, in which case we
always update.

The old > check got this wrong when our version was odd (e.g. 3) and the
reply version was even (e.g. 2): the older stale (v2) info would be
applied.  Fixed and clarified the comment.

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