kernel/pipe: implement direct-to-pending-readers data copy

If there are pending readers, it is best to perform a single data copy
directly into their final destination buffer rather than doing one copy
into the ring buffer just to immediately copy the same data out of it.

Incidentally, this allows for supporting pipes with no ring buffer at all.

The pipe implementation being deprecated has a similar capability so better
have it here too.

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