copy_xstate_to_kernel(): don't leave parts of destination uninitialized

commit 9e4636545933131de15e1ecd06733538ae939b2f upstream.

copy the corresponding pieces of init_fpstate into the gaps instead.

Cc: stable@kernel.org
Tested-by: Alexander Potapenko <glider@google.com>
Acked-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>