nfsd: Remove unnecessary NULL checks

"cb" is never actually NULL in these functions.

On a quick skim of the history, they seem to have been there from the
beginning.  I'm not sure if they originally served a purpose.

Reported-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>