mm/memcg: set pos to prev unconditionally

mm/memcg: set pos to prev unconditionally

Current code sets pos to prev based on condition (prev && !reclaim), while
we can do this unconditionally.

Since:

  * If !reclaim, pos is the same as prev no matter it is NULL or not.
  * If reclaim, pos would be set properly from iter->position.

Link: https://lkml.kernel.org/r/20220225003437.12620-3-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>