The PERF_EVENT_IOC_PERIOD ioctl command can be used to change the
sample period of a running perf_event. Consequently, when calculating
the next event period, the new period will only be considered after the
previous one has overflowed.

This patch changes the calculation of the remaining event ticks so that
they are offset if the period has changed.

Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Reported-by: Andreas Sandberg <andreas.sandberg@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>