mm: swap_cgroup: allocate swap_cgroup map using vcalloc()

mm: swap_cgroup: allocate swap_cgroup map using vcalloc()

Currently swap_cgroup's map is constructed as a vmalloc()'s-based array of
pointers to individual struct pages.  This brings an unnecessary
complexity into the code.

This patch turns the swap_cgroup's map into a single space
allocated by vcalloc().

[akpm@linux-foundation.org: s/vfree/kvfree/, per Shakeel]
Link: https://lkml.kernel.org/r/20241115190229.676440-1-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>