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/[e-mail]/

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 <[e-mail]>
Signed-off-by: Mateusz Guzik <[e-mail]>
Reviewed-by: Kuniyuki Iwashima <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/unix/garbage.c

···
 	}
 }
 
-static DEFINE_SPINLOCK(unix_gc_lock);
+static __cacheline_aligned_in_smp DEFINE_SPINLOCK(unix_gc_lock);
 
 void unix_add_edges(struct scm_fp_list *fpl, struct unix_sock *receiver)
 {