f2fs: fix to correct return value of f2fs_trim_fs

We should account trimmed block number from __wait_all_discard_cmd
in __issue_discard_cmd_range, otherwise trimmed blocks returned
by f2fs_trim_fs will be wrong, this patch fixes it.

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