Revert "LU-3333 ptlrpc: Protect request buffer changing"

Revert "LU-3333 ptlrpc: Protect request buffer changing"

It turns out that sec_null message format is is not double wrapped
like sec_plain and sec_gss cases. As such a conversion from rq_buf
to rq_msg is different in those cases.
The older way of doing in-place locking seems to be the way to go
here I think.

This reverts commit b2bb3b247d1dc75e25f1b5c14a333905909b5e70.

Change-Id: Icd84a696aafed17a275d0576dad639b929618190
Reviewed-on: http://review.whamcloud.com/10052
Tested-by: Jenkins
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Tested-by: Oleg Drokin <oleg.drokin@intel.com>