afs: casting NULL is generally unnecessary

afs: casting NULL is generally unnecessary

Casting NULL shouldn't be necessary in most cases.  Eliminate this
in favor of shorter lines and consistency.  Function pointers, variadic
arguments should still be cast.

Change-Id: Ibcd67378556e45c2b24f7aa85a4d73c72cd88e02
Reviewed-on: http://gerrit.openafs.org/8151
Reviewed-by: Garrett Wollman <wollman@csail.mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>