Bug 1518762 - Avoid error in psutil.disk_io_counters in resourcemonit…

…or. r=ahal, a=jcristau DONTBUILD

Imports giampaolo/psutil@8f99f37
to fix a bug with Linux kernel 4.18+, giampaolo/psutil#1354
(to be included in a future release of psutil, version 5.5.0)

Differential Revision: https://phabricator.services.mozilla.com/D16028

[jcristau: re-applying this change which was accidentally reverted in
 https://hg.mozilla.org/releases/mozilla-release/rev/e2030db2e502]

third_party/python/psutil/psutil/_pslinux.py

@@ -1051,6 +1051,8 @@ def get_partitions():
         # ...unless (Linux 2.6) the line refers to a partition instead
         # of a disk, in which case the line has less fields (7):
         # "3    1   hda1 8 8 8 8"
+        # 4.18+ has 4 fields added:
+        # "3    0   hda 8 8 8 8 8 8 8 8 8 8 8 0 0 0 0"
         # See:
         # https://www.kernel.org/doc/Documentation/iostats.txt
         # https://www.kernel.org/doc/Documentation/ABI/testing/procfs-diskstats
@@ -1062,7 +1064,7 @@ def get_partitions():
             reads = int(fields[2])
             (reads_merged, rbytes, rtime, writes, writes_merged,
                 wbytes, wtime, _, busy_time, _) = map(int, fields[4:14])
-        elif fields_len == 14:
+        elif fields_len == 14 or fields_len == 18:
             # Linux 2.6+, line referring to a disk
             name = fields[2]
             (reads, reads_merged, rbytes, rtime, writes, writes_merged,