sunrpc: have svc_wake_up only deal with pool 0

The way that svc_wake_up works is a bit inefficient. It walks all of the
available pools for a service and either wakes up a task in each one or
sets the SP_TASK_PENDING flag in each one.

When svc_wake_up is called, there is no need to wake up more than one
thread to do this work. In practice, only lockd currently uses this
function and it's single threaded anyway. Thus, this just boils down to
doing a wake up of a thread in pool 0 or setting a single flag.

Eliminate the for loop in this function and change it to just operate on
pool 0. Also update the comments that sit above it and get rid of some
code that has been commented out for years now.

Signed-off-by: Jeff Layton <jlayton@primarydata.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>