nfs: simplify two-level sysctl registration for nfs_cb_sysctls

There is no need to declare two tables to just create directories,
this can be easily be done with a prefix path with register_sysctl().

Simplify this registration.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>