mm: memcg: declare do_memsw_account inline

In commit 66d60c4 ("mm: memcg: move legacy memcg event code into
memcontrol-v1.c"), the static do_memsw_account() function was moved from a
.c file to a .h file.  Unfortunately, the traditional inline keyword
wasn't added.  If a file (e.g., a unit test) includes the .h file, but
doesn't refer to do_memsw_account(), it will get a warning like:

mm/memcontrol-v1.h:41:13: warning: unused function 'do_memsw_account' [-Wunused-function]
   41 | static bool do_memsw_account(void)
      |             ^~~~~~~~~~~~~~~~

Link: https://lkml.kernel.org/r/20241128203959.726527-1-jsperbeck@google.com
Fixes: 66d60c4 ("mm: memcg: move legacy memcg event code into memcontrol-v1.c")
Signed-off-by: John Sperbeck <jsperbeck@google.com>
Acked-by: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: John Sperbeck

mm/memcontrol-v1.h

@@ -38,7 +38,7 @@ void mem_cgroup_id_put_many(struct mem_cgroup *memcg, unsigned int n);
 	     iter = mem_cgroup_iter(NULL, iter, NULL))
 
 /* Whether legacy memory+swap accounting is active */
-static bool do_memsw_account(void)
+static inline bool do_memsw_account(void)
 {
 	return !cgroup_subsys_on_dfl(memory_cgrp_subsys);
 }