nfs: Remove unnecessary NULL check before kfree()

Since kfree() already checks if its argument is NULL, an additional
check before calling kfree() is unnecessary and can be removed.

Remove it and thus also the following Coccinelle/coccicheck warning
reported by ifnullfree.cocci:

  WARNING: NULL check before some freeing functions is not needed

Reviewed-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>