LU-2936 ptlrpc: Do not try to fetch hp request blindly

LU-2936 ptlrpc: Do not try to fetch hp request blindly

ptlrpc_svcpt_health_check tries to blindly fetch a hp requet from
any service it happens to be called on, but some services don't have
any hp policies registered resulting in an underlyign assertion
in nrs_svcpt2nrs.
Make sure there are in fact pending hp requests on a service before
attempting to fetch them.

Change-Id: Ia38dcce758db948a1e4c187d009da4a8d5f2cbc6
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Reviewed-on: http://review.whamcloud.com/5665
Tested-by: Hudson
Reviewed-by: Nikitas Angelinas <nikitas_angelinas@xyratex.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Liang Zhen <liang.zhen@intel.com>