f2fs: fix freezing filesystem during resize

f2fs: fix freezing filesystem during resize

Using FREEZE_HOLDER_USERSPACE has two consequences:

(1) If userspace freezes the filesystem after mnt_drop_write_file() but
    before freeze_super() was called filesystem resizing will fail
    because the freeze isn't marked as nestable.

(2) If the kernel has successfully frozen the filesystem via
    FREEZE_HOLDER_USERSPACE userspace can simply undo it by using the
    FITHAW ioctl.

Fix both issues by using FREEZE_HOLDER_KERNEL. It will nest with
FREEZE_HOLDER_USERSPACE and cannot be undone by userspace.

And it is the correct thing to do because the kernel temporarily freezes
the filesystem.

Signed-off-by: Christian Brauner <brauner@kernel.org>