f2fs: fix to issue small discard in real-time mode discard

Now in f2fs, we share functions and structures for batch mode and real-time mode
discard. For real-time mode discard, in shared function add_discard_addrs, we
will use uninitialized trim_minlen in struct cp_control to compare with length
of contiguous free blocks to decide whether skipping discard fragmented freespace
or not, this makes us ignore small discard sometimes. Fix it.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Reviewed-by : Changman Lee <cm224.lee@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>