perf tools: Fix possible compiler warnings in hashmap

perf tools: Fix possible compiler warnings in hashmap

The hashmap__for_each_entry[_safe] is accessing 'map' as if it's a
pointer.  But it does without parentheses so passing a static hash map
with an ampersand (like &slab_hash below) caused compiler warnings due
to unmatched types.

  In file included from util/bpf_lock_contention.c:5:
  util/bpf_lock_contention.c: In function ‘exit_slab_cache_iter’:
  linux/tools/perf/util/hashmap.h:169:32: error: invalid type argument of ‘->’ (have ‘struct hashmap’)
    169 |         for (bkt = 0; bkt < map->cap; bkt++)                                \
        |                                ^~
  util/bpf_lock_contention.c:105:9: note: in expansion of macro ‘hashmap__for_each_entry’
    105 |         hashmap__for_each_entry(&slab_hash, cur, bkt)
        |         ^~~~~~~~~~~~~~~~~~~~~~~
  /home/namhyung/project/linux/tools/perf/util/hashmap.h:170:31: error: invalid type argument of ‘->’ (have ‘struct hashmap’)
    170 |                 for (cur = map->buckets[bkt]; cur; cur = cur->next)
        |                               ^~
  util/bpf_lock_contention.c:105:9: note: in expansion of macro ‘hashmap__for_each_entry’
    105 |         hashmap__for_each_entry(&slab_hash, cur, bkt)
        |         ^~~~~~~~~~~~~~~~~~~~~~~

Cc: bpf@vger.kernel.org
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20241009202009.884884-1-namhyung@kernel.org
Signed-off-by: Namhyung Kim <namhyung@kernel.org>