dm persistent data: packed struct should have an aligned() attribute too

Otherwise most non-x86 architectures (e.g. riscv, arm) will resort to
byte-by-byte access.

Cc: stable@vger.kernel.org
Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>