LU-2467 ptlrpc: Fix an unideal symbol export

LU-2467 ptlrpc: Fix an unideal symbol export

The "suppress_pings" symbol is a little bit too short to export.  This
patch exports a function, ptlrpc_pinger_suppress_pings(), instead.

Change-Id: Ifdcf5d2459baa7ae2709572a2fd4d02b72e440df
Signed-off-by: Li Wei <wei.g.li@intel.com>
Reviewed-on: http://review.whamcloud.com/5270
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Emoly Liu <emoly.liu@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>