f2fs: cleanup duplicate stats for atomic files

Remove duplicate sbi->aw_cnt stats counter that tracks
the number of atomic files currently opened (it also shows
incorrect value sometimes). Use more relit lable sbi->atomic_files
to show in the stats.

Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>