LU-543 mdd: fix rename changelog

LU-543 mdd: fix rename changelog

Current rename changelog stores source fid in both CL_RENAME & CL_EXT
records, which is redundant, and the 'tfid' in CL_EXT is never been
used.

Actually, we'd store target fid in the CL_EXT record, then application
could detect the fid unlinked by rename in changelog.

Signed-off-by: Niu Yawei <niu@whamcloud.com>
Change-Id: I0c616f813657a2faefa60a707f4fc1d9dc971b39
Reviewed-on: http://review.whamcloud.com/1652
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Mikhail Pershin <tappro@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>