LU-2714 hsm: limit MDT-side allocations for HSM RPCs

Limit the amount of memory an MDT will allocate for a single HSM RPC
to 1 MB and add some sanity checking to the HSM handlers that use
variable length buffers.  In hur_len() compute the size of an HSM
request in a portable way.

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: Ie03b85a8524cb377bf43446be429cc60c2fe39a7
Reviewed-on: http://review.whamcloud.com/5507
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>