sunrpc: simplify two-level sysctl registration for svcrdma_parm_table

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.

Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>