mds: Fix MMDSCacheRejoin::dn_strong version check failure

MMDSCacheRejoin::dn_strong structure inside MMDSCacheRejoin
class is not versioned. But the referent inodes feature
requires to add a new variable to 'dn_strong'. Since
it's not versioned, the ceph-object-corpus encode/decode
test (src//readable.sh) is failing for no backward
compatibility.

This patch fixes the versioning issue of MMDSCacheRejoin::dn_strong
by introducing a new struct MMDSCacheRejoin::dn_strong_new
which encapsulates 'dn_strong' and the new variable and is versioned.

Signed-off-by: Kotresh HR <[email protected]>

Author: kotreshhr