Cleanup.

Just store cache_detail in seq_file's private,
an allocated handle is redundant.

v8, same as v6.
Signed-off-by: NKinglong Mee <kinglongmee@gmail.com>
Reviewed-by: NNeilBrown <neilb@suse.com>
Signed-off-by: NJ. Bruce Fields <bfields@redhat.com>