Simplify and optimize __fill_random_buf

Simplify and optimize __fill_random_buf

This reduces the number of source lines and the code size.

For example, when compiling with GCC 12.1 (-O3 -march=skylake), the
resulting assembly shrinks from 33 to 27 instructions and the number of
jump instructions is reduced from 4 to 3.

NB: GCC is able to eliminate the memcpy() call.

NB: Even if a compiler doesn't eliminate the memcpy() call, it's very
unlikely to ever get called since the buffer sizes are expected to be
powers of two (>= 8), usually.

Signed-off-by: Georg Sauthoff <mail@gms.tf>