SUNRPC: relocate the creation of socket-specific tunables

Clean-up:

The RPC client currently creates some sysctls that are specific to the
socket transport.  Move those entirely into xprtsock.c.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>