Update git submodules

Update git submodules

* Update docs/submodules/samplevnf from branch 'master'
  - Merge "Fix minimum latency"
  - Fix minimum latency

    Revert back part of commit 9fa316261d7d9
    The function abs_diff was erroneously changed to diff_or_zero.
    This function was supposed to measure the difference between rx and
    tx time; when rx time overflowed and tx time not yet (i.e.
    rx time ~= 0 and tx_time ~=UINT32_MAX, this function added UINT32_MAX
    to rx_time. The name of the function was confusing and caused the
    previous commit. Net effect of previous commit was that every four seconds
    the minimum latency was 0
    This commit reverse back to the original behavior, with a function name
    diff_time.

    Change-Id: Ia1b80e48a756cf5df411dcf58ca1cbc835214d13
    Signed-off-by: Xavier Simonart <xavier.simonart@intel.com>