f2fs: fix to check return value of f2fs_do_truncate_blocks()

Otherwise, if truncation on cow_inode failed, remained data may
pollute current transaction of atomic write.

Cc: Daeho Jeong <daehojeong@google.com>
Fixes: a46bebd5 ("f2fs: synchronize atomic write aborts")
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>