LU-398 ptlrpc: NRS framework follow-up patch

LU-398 ptlrpc: NRS framework follow-up patch

This patch addresses some outstanding issues that had been raised
by reviewers of the "Add the NRS framework and FIFO policy" patch,
and include some other improvements, e.g. it reworks the API
slightly in order to optimize some frequently-used operations,
does not uanncessarily policies in liblustre and client-only
kernel builds, and makes sure we hold module references when
required, for policies registering from other modules.

Signed-off-by: Nikitas Angelinas <nikitas_angelinas@xyratex.com>
Change-Id: I9306d43e2aef20aa64d6870a56ae99859ce40cd5
Oracle-bug-id: b=13634
Xyratex-bug-id: MRP-73
Reviewed-on: http://review.whamcloud.com/5274
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Liang Zhen <liang.zhen@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>