iolog: get rid of one section of CONFIG_ZLIB ifdef

Signed-off-by: Jens Axboe <axboe@fb.com>
iolog: get rid of ic->nofree

Push the free out of flush_chunk(), then the caller just does it
instead. Unifies the log write vs compress as well.

Signed-off-by: Jens Axboe <axboe@fb.com>
iolog: use a define instead of a raw bit mask

Add a comment too, while in there.

Signed-off-by: Jens Axboe <axboe@fb.com>
flist: add flist_first_entry()

Signed-off-by: Jens Axboe <axboe@fb.com>