mm/memcg: move generation assignment and comparison together

mm/memcg: move generation assignment and comparison together

For each round-trip, we assign generation on first invocation and compare
it on subsequent invocations.

Let's move them together to make it more self-explaining.  Also this
reduces a check on prev.

Link: https://lkml.kernel.org/r/20220225003437.12620-4-richard.weiyang@gmail.com
Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>