f2fs: switch to the new mount api

The new mount api will execute .parse_param, .init_fs_context, .get_tree
and will call .remount if remount happened. So we add the necessary
functions for the fs_context_operations. If .init_fs_context is added,
the old .mount should remove.

See Documentation/filesystems/mount_api.rst for more information.

Signed-off-by: Hongbo Li <lihongbo22@huawei.com>
[sandeen: forward port]
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
[hongbo: context modified]
Signed-off-by: Hongbo Li <lihongbo22@huawei.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>