mm: swap_cgroup: get rid of __lookup_swap_cgroup()

mm: swap_cgroup: get rid of __lookup_swap_cgroup()

Because swap_cgroup map is now virtually contiguous, swap_cgroup_record()
can be simplified, which eliminates a need to use __lookup_swap_cgroup().

Now as __lookup_swap_cgroup() is really trivial and is used only once, it
can be inlined.

Link: https://lkml.kernel.org/r/20241115190229.676440-2-roman.gushchin@linux.dev
Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>