LU-56 ptlrpc: common code to validate nthreads

LU-56 ptlrpc: common code to validate nthreads

Add a common function to validate ptlrpc service threads number,
instead of checking it before registration of each service.
This function (ptlrpc_server_nthreads_check) will be enhanced
soon for CPT affinity ptlrpc service.

Signed-off-by: Liang Zhen <liang@whamcloud.com>
Change-Id: I5db09396f4a95e36d8f5d38777c51eafb7dea5b1
Reviewed-on: http://review.whamcloud.com/2879
Tested-by: Hudson
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: wangdi <di.wang@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>