LU-11414 ptlrpc: don't change buffer when signature is ready

LU-11414 ptlrpc: don't change buffer when signature is ready

The lm_repsize is part of buffer being used in signature calculation
and must not be changed after calculation is done.

Patch reverts related changes from commit 13372d6c and moves related
lm_repsize update into MDC where DOM read-on-open buffer is prepared

Signed-off-by: Mikhail Pershin <mpershin@whamcloud.com>
Change-Id: Ic277ca26ad8fb8e23ee7e4b6e81d84a1d51b4733
Reviewed-on: https://review.whamcloud.com/33223
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>