LU-3285 mdt: remove assertion on mdt_pack_size2body()

LU-3285 mdt: remove assertion on mdt_pack_size2body()

The assertion on result of mdt_pack_size2body() is not
always correct, e.g. in case of resend.

Test-Parameters: mdssizegb=20 testlist=sanity-dom,dom-performance
Signed-off-by: Mikhal Pershin <mike.pershin@intel.com>
Change-Id: Iff2c9723959db591fdd29074b0186d8ddf0752e9
Reviewed-on: https://review.whamcloud.com/30058
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>