SUNRPC: Fix possible autodisconnect during connect due to old last_used

Ensure last_used is updated before calling mod_timer inside
xprt_schedule_autodisconnect.  This avoids a possible xprt_autoclose
firing immediately after a successful connect when xprt_unlock_connect
calls xprt_schedule_autodisconnect with an old value of last_used.

Signed-off-by: Dave Wysochanski <dwysocha@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>