kernel/pipe: disable direct-to-readers copy with CONFIG_KERNEL_COHERENCE

Systems that enabled this option don't have their stacks in coherent
memory. Given our pipe_buf_spec is stored on the stack, and readers may
also have their destination buffer on their stack too, it is not worth
going to the trouble of supporting direct-to-readers copy with them.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>