Fix alignment of buffer used for dm_ioctl.

Fix alignment of buffer used for dm_ioctl.

Since the dm_ioctl struct was being allocated on the stack as a large
character array, it was getting character alignment rather than the
proper alignment for the struct. GCC had been getting away with this
so far, but it's undefined behavior that clang managed to expose.

Bug: 18736778
Change-Id: Ied275dfad7fcc41d712b2d02c8a185f499221f57