perf/x86/lbr: Fix shorter LBRs call stacks for the system-wide mode

perf/x86/lbr: Fix shorter LBRs call stacks for the system-wide mode

In the system-wide mode, LBR callstacks are shorter in comparison to
the per-process mode.

LBR MSRs are reset during a context switch in the system-wide mode. For
the LBR call stack, the LBRs should be always saved/restored during a
context switch.

Use the space in task_struct to save/restore the LBR call stack data.

For a system-wide event, it's unnecessagy to update the
lbr_callstack_users for each threads. Add a variable in x86_pmu to
indicate whether the system-wide event is active.

Fixes: 76cb2c617f12 ("perf/x86/intel: Save/restore LBR stack during context switch")
Reported-by: Andi Kleen <ak@linux.intel.com>
Reported-by: Alexey Budankov <alexey.budankov@linux.intel.com>
Debugged-by: Alexey Budankov <alexey.budankov@linux.intel.com>
Signed-off-by: Kan Liang <kan.liang@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250314172700.438923-5-kan.liang@linux.intel.com