LU-4877 mdt: tolerate zero md_packed in mdt_fix_reply

LU-4877 mdt: tolerate zero md_packed in mdt_fix_reply

The assertion for md_packed value is not really needed there,
if it is zero then buffer is not packed in reply and is not
needed, it is enough just unmark big buffer usage.

Signed-off-by: Mikhail Pershin <mike.pershin@intel.com>
Change-Id: I4db92ce2ef492069db4dbd4d07c0d69770130fad
Reviewed-on: http://review.whamcloud.com/10116
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: wangdi <di.wang@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>