sunrpc: Simplify do_enqueue tracing

There are three cases where svc_xprt_do_enqueue() returns without
waking an nfsd thread:

1. There is no work to do

2. The transport is already busy

3. There are no available nfsd threads

Only 3. is truly interesting. Move the trace point so it records
that there was work to do and either an nfsd thread was awoken, or
a free one could not found.

As an additional clean up, remove a redundant comment and a couple
of dprintk call sites.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>