f2fs: Do not issue small discards in LFS mode

clear_prefree_segments() issues small discards after discarding full
segments. These small discards may not be section aligned, so not zone
aligned on a zoned block device, causing __f2fs_iissue_discard_zone() to fail.
Fix this by not issuing small discards for a volume mounted with the BLKZONED
feature enabled.

Cc: stable@vger.kernel.org
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>