arm64: perf: Don't register user access sysctl handler multiple times

Commit e2012600 ("arm64: perf: Add userspace counter access disable
switch") introduced a new 'perf_user_access' sysctl file to enable and
disable direct userspace access to the PMU counters. Sadly, Geert
reports that on his big.LITTLE SoC ('Renesas Salvator-XS w/ R-Car H3'),
the file is created for each PMU type probed, resulting in a splat
during boot:

  | hw perfevents: enabled with armv8_cortex_a53 PMU driver, 7 counters available
  | sysctl duplicate entry: /kernel//perf_user_access
  | CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.16.0-rc3-arm64-renesas-00003-ge2012600810c #1420
  | Hardware name: Renesas Salvator-X 2nd version board based on r8a77951 (DT)
  | Call trace:
  |  dump_backtrace+0x0/0x190
  |  show_stack+0x14/0x20
  |  dump_stack_lvl+0x88/0xb0
  |  dump_stack+0x14/0x2c
  |  __register_sysctl_table+0x384/0x818
  |  register_sysctl+0x20/0x28
  |  armv8_pmu_init.constprop.0+0x118/0x150
  |  armv8_a57_pmu_init+0x1c/0x28
  |  arm_pmu_device_probe+0x1b4/0x558
  |  armv8_pmu_device_probe+0x18/0x20
  |  platform_probe+0x64/0xd0
  |  hw perfevents: enabled with armv8_cortex_a57 PMU driver, 7 counters available

Introduce a state variable to track creation of the sysctl file and
ensure that it is only created once.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Fixes: e2012600 ("arm64: perf: Add userspace counter access disable switch")
Link: https://lore.kernel.org/r/CAMuHMdVcDxR9sGzc5pcnORiotonERBgc6dsXZXMd6wTvLGA9iw@mail.gmail.com


Signed-off-by: Will Deacon <will@kernel.org>