dma-buf: Use monotonic clock for buffer time tracking

gettimeofday() uses CLOCK_REALTIME which gives the current real-world
time. This time is discontiguous from boot till the userspace sets
the "wall clock time". Depending on such discountigous timer for
tracking buffer alive time is not right. Instead, use ktime which
uses CLOCK_MONOTONIC which is guaranteed to increase monotonically
from zero when system boots.

Change-Id: I2889be4ac7bb0e2ffd8845af1a8c91759712c891
Signed-off-by: Sudarshan Rajagopalan <sudaraja@codeaurora.org>