nfs: update for module_param_named API change

After merging the rr tree, today's linux-next build (powerpc
ppc64_defconfig) failed like this:

net/sunrpc/auth.c:74: error: 'param_ops_hashtbl_sz' undeclared here (not in a function)

Caused by commit 0685652df0929cec7d78efa85127f6eb34962132
("param:param_ops") interacting with commit
f8f853ab19fcc415b6eadd273373edc424916212 ("SUNRPC: Make the credential
cache hashtable size configurable") from the nfs tree.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>