perf: Rename perf_event_context.nr_pending to nr_no_switch_fast.

nr_pending counts the number of events in the context that
either pending_sigtrap or pending_work, but it is used
to prevent taking the fast path in perf_event_context_sched_out.

Renamed to reflect what it is used for, rather than what it
counts. This change allows using the field to track other
event properties that also require skipping the fast path
without possible confusion over the name.

Signed-off-by: Ben Gainey <ben.gainey@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20240730084417.7693-2-ben.gainey@arm.com