net: sockets: socketpair: do not allow blocking IO in ISR context

Using a socketpair for communication in an ISR is not a great
solution, but the implementation should be robust in that case
as well.

It is not acceptible to block in ISR context, so robustness here
means to return -1 to indicate an error, setting errno to `EAGAIN`
(which is synonymous with `EWOULDBLOCK`).

Fixes #25417

Signed-off-by: Chris Friedt <cfriedt@meta.com>