memcg: simple cleanup of stats update functions

mod_memcg_lruvec_state() is never called from outside of memcontrol.c and
with always irq disabled.  So, replace it with the irq disabled version
and add an assert that irq is disabled in the caller.

Similarly mod_objcg_state() is not called from outside of memcontrol.c, so
simply make it static and change it's name to __mod_objcg_state().

Link: https://lkml.kernel.org/r/20240420232505.2768428-1-shakeel.butt@linux.dev
Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: T.J. Mercier <tjmercier@google.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>