OProfile: Fix buffer synchronization for IBS

The patch is needed since there is some IBS code in add_ibs_begin()
that handles more than one sample per iteration. This requires calling
get_slots() during each loop.

This fixes the current problem, but a proper solution that reworks the
cpu buffer synchronization is needed here in the future.

Signed-off-by: Barry Kasindorf <barry.kasindorf@amd.com>
Signed-off-by: Robert Richter <robert.richter@amd.com>