af_unix: annotate unix_gc_lock with __cacheline_aligned_in_smp

Otherwise the lock is susceptible to ever-changing false-sharing due to
unrelated changes. This in particular popped up here where an unrelated
change improved performance:
https://lore.kernel.org/oe-lkp/202511281306.51105b46-lkp@intel.com/

Stabilize it with an explicit annotation which also has a side effect
of furher improving scalability:
> in our oiginal report, 284922f4c5 has a 6.1% performance improvement comparing
> to parent 17d85f33a8.
> we applied your patch directly upon 284922f4c5. as below, now by
> "284922f4c5 + your patch"
> we observe a 12.8% performance improvements (still comparing to 17d85f33a8).

Note nothing was done for the other fields, so some fluctuation is still
possible.

Tested-by: kernel test robot <oliver.sang@intel.com>
Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20251203100122.291550-1-mjguzik@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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