SUNRPC: Move initialization of rq_stime

Micro-optimization: Call ktime_get() only when ->xpo_recvfrom() has
given us a full RPC message to process. rq_stime isn't used
otherwise, so this avoids pointless work.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Acked-by: Tom Talpey <tom@talpey.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>