In multi-window mode, the read iterator is supposed to start from the
window with the oldest data, which is, chronologically, the next window
after the one with the newest data. This, however, fails to take into
account the potentially empty windows, so in short trace sessions it's
possible to have a lot of zeroes read from the character device first.

Fix this by skipping over the empty windows in initialization of the
read iterator.
Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20190627125152.54905-5-alexander.shishkin@linux.intel.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>