LU-4423 ptlrpc: fix potential NULL pointer dereference

The rest of the code seem to imply that rmf_dumper may indeed be
NULL.  Change the code so that dumping is not even considered if
rmf_dumper callback is not set.

Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>

Change-Id: Iaea16aaf799976d08ebb51322021cc879db1c6d8
Reviewed-on: http://review.whamcloud.com/8682
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Emoly Liu <emoly.liu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>