mds: journal original dirfrags for rollback

mds: journal original dirfrags for rollback

Fragments with different 'bits' can be merged into one fragment. So we can't
use 'basefrag' and 'bits' to calculate the original fragments when rolling
back a merge operation.

We also can't rely on MDCache::adjust_dir_fragments() to restore the original
fragments' statistics during rolling back. This is because fragments are not
always complete in the rollback case.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>