f2fs: fix to release discard entries during checkpoint

In f2fs_fill_super, if there is any IO error occurs during recovery,
cached discard entries will be leaked, in order to avoid this, make
write_checkpoint() handle memory release by itself, besides, move
clear_prefree_segments to write_checkpoint for readability.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>