Let's create /sys/fs/f2fs/<disk>/feature_list/ to meet sysfs rule.

Note that there are three feature list entries:
1) /sys/fs/f2fs/features
  : shows runtime features supported by in-kernel f2fs along with Kconfig.
    - ref. F2FS_FEATURE_RO_ATTR()

2) /sys/fs/f2fs/$s_id/features <deprecated>
  : shows on-disk features enabled by mkfs.f2fs, used for old kernels. This
    won't add new feature anymore, and thus, users should check entries in 3)
    instead of this 2).

3) /sys/fs/f2fs/$s_id/feature_list
  : shows on-disk features enabled by mkfs.f2fs per instance, which follows
    sysfs entry rule where each entry should expose single value.
    This list covers old feature list provided by 2) and beyond. Therefore,
    please add new on-disk feature in this list only.
    - ref. F2FS_SB_FEATURE_RO_ATTR()
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>