staging: lustre: 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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>