SUNRPC: rename some functions from rqst_ to svc_thread_

Functions which directly manipulate a 'struct rqst', such as
svc_rqst_alloc() or svc_rqst_release_pages(), can reasonably
have "rqst" in there name.
However functions that act on the running thread, such as
XX_should_sleep() or XX_wait_for_work() should seem more
natural with a "svc_thread_" prefix.

So make those changes.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>