memcg: do not check for do_swap_account in mem_cgroup_{read,write,reset}

memcg: do not check for do_swap_account in mem_cgroup_{read,write,reset}

Since 2d11085e ("memcg: do not create memsw files if swap accounting is
disabled") memsw files are created only if memcg swap accounting is
enabled so it doesn't make any sense to check for it explicitly in
mem_cgroup_read(), mem_cgroup_write() and mem_cgroup_reset().

Signed-off-by: Michal Hocko <mhocko@suse.cz>
Cc: Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>