net: socketpair to use ring_buffer instead of k_pipe

Replaced the k_pipe-based implementation in sockpair with ring_buffer
based implementation instead.
The move to ring_buffer is done to avoid overhead of k_pipe and to align
with the new k_pipe API.
This does not pose any added risk to concurrency as the read and write
functions are protected by semaphores for both spairs.

Signed-off-by: Måns Ansgariusson <Mansgariusson@gmail.com>