cgroup: unify read path so that seq_file is always used

With the recent removal of cftype->read() and ->read_map(), only three
operations are remaining, ->read_u64(), ->read_s64() and
->read_seq_string().  Currently, the first two are handled directly
while the last is handled through seq_file.

It is trivial to serve the first two through the seq_file path too.
This patch restructures read path so that all operations are served
through cgroup_seqfile_show().  This makes all cgroup files seq_file -
single_open/release() are now used by default,
cgroup_seqfile_operations is dropped, and cgroup_file_operations uses
seq_read() for read.

This simplifies the code and makes the read path easy to convert to
use kernfs.

Note that, while cgroup_file_operations uses seq_read() for read, it
still uses generic_file_llseek() for seeking instead of seq_lseek().
This is different from cgroup_seqfile_operations but shouldn't break
anything and brings the seeking behavior aligned with kernfs.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>