bus: align kdbus ioctl parameters to 8byte

bus: align kdbus ioctl parameters to 8byte

All kdbus ioctl arguments must be 8byte aligned. Make sure we use
alloca_align() and _alignas_(8) in all situations where gcc doesn't
guarantee 8-byte alignment.

Note that objects on the stack are always 8byte aligned as we put
_alignas_(8) into the structure definition in kdbus.h.