[Linux] disk_io_counters() ValueError when parsing /sys/block

[giampaolo]

Fixes:

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ERROR: psutil.tests.test_linux.TestSystemDiskIoCounters.test_emulate_use_sysfs
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Traceback (most recent call last):
  File "/home/giampaolo/svn/psutil/psutil/tests/test_linux.py", line 1195, in test_emulate_use_sysfs
    wsysfs = psutil.disk_io_counters(perdisk=True)
  File "/home/giampaolo/svn/psutil/psutil/__init__.py", line 2065, in disk_io_counters
    rawdict = _psplatform.disk_io_counters(**kwargs)
  File "/home/giampaolo/svn/psutil/psutil/_pslinux.py", line 1124, in disk_io_counters
    for entry in gen:
  File "/home/giampaolo/svn/psutil/psutil/_pslinux.py", line 1110, in read_sysfs
    wbytes, wtime, _, busy_time, _) = map(int, fields)
ValueError: too many values to unpack (expected 11)