LU-5396 all: use NULL instead of 0

LU-5396 all: use NULL instead of 0

It is preferable to use NULL instead of 0 for pointers. This fixes sparse
warnings such as:

  lustre/fld/fld_request.c:126:17: warning: Using plain integer as NULL pointer

The second parameter of class_match_param() was changed to a const, to
be able to remove a cast in one user, to prevent splitting a long
line. No other code change.

Change-Id: Icc7b46cef32685c942f5af970d1610467fee4496
Signed-off-by: frank zago <fzago@cray.com>
Reviewed-on: http://review.whamcloud.com/12567
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>