f2fs: fix to return 0 if err == -ENOENT in f2fs_readdir

Commit 57b62d29 ("f2fs: fix to report
error in f2fs_readdir") causes f2fs_readdir to return -ENOENT when
get_lock_data_page returns -ENOENT. However, the original logic is to
continue when get_lock_data_page returns -ENOENT, but it forgets to
reset err to 0.

This will cause getdents64 incorretly return -ENOENT when lastdirent is
NULL in getdents64. This will lead to a wrong return value for syscall
caller.

Signed-off-by: Yunlong Song <yunlong.song@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>