SUNRPC: Micro-optimise when the task is known not to be sleeping

In cases where we know the task is not sleeping, try to optimise
away the indirect call to task->tk_action() by replacing it with
a direct call.
Only change tail calls, to allow gcc to perform tail call
elimination.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>