Wall time obtained from ktime_get_real_ns is susceptible to sudden jumps due to
user setting the time or due to NTP.  Boot time is constantly increasing time
better suited for comparing two timestamps.
Signed-off-by: NAbhilash Jindal <klock.android@gmail.com>
Reviewed-by: NHarald Geyer <harald@ccbib.org>
Signed-off-by: NJonathan Cameron <jic23@kernel.org>