SUNRPC: Create a helper function for accessing the rpc_clnt's xprt_switch

This function takes the necessary rcu read lock to dereference the
client's rpc_xprt_switch and bump the reference count so it doesn't
disappear underneath us before returning. This does mean that callers
are responsible for calling xprt_switch_put() on the returned object
when they are done with it.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>