selftests/powerpc/pmu/ebb: fix lost_exception_test hang with sched yield change

commit 79104becf42b ("sched/fair: Forfeit vruntime on yield") changed
yield_task_fair() to only bump the deadline when the entity is eligible
(vruntime <= avg_vruntime). When the entity is ineligible the yield
becomes a complete no-op from scheduling perspective.

lost_exception_test calls sched_yield() 100,000 times per iteration to
race the EBB exception delivery with a context switch to the eat_cpu
companion process. After enough iterations the test process's vruntime
races ahead of avg_vruntime (each eligible yield bumps vruntime to
deadline, then advances deadline by one slice). Once ineligible,
yield_task_fair() does nothing: so the scheduler won't pick the
eat_cpu child. No context switch occurs, the PMAO race is never
triggered, and ebb_count stays at 0 forever causing the test to
hang until timeout.

Fix by replacing sched_yield() with nanosleep(0, 1ns). nanosleep() goes
through hrtimer_nanosleep() -> do_nanosleep(), which puts the task into
TASK_INTERRUPTIBLE and removes it from the run queue entirely. This
guarantees the scheduler picks the eat_cpu child, restoring the
context-switch guarantee the test requires. The 1ns duration is enough
to engage the hrtimer path while keeping the sleep effectively
instantaneous; the same race window between PMU overflow and context
switch is preserved.

Reported-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Signed-off-by: Athira Rajeev <atrajeev@linux.ibm.com>
Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20260903074036.63309-1-atrajeev@linux.ibm.com

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