socket: downgrade log warnings about inability to set socket buffer sizes

In containers we might lack the privs to up the socket buffers. Let's
not complain so loudly about that. Let's hence downgrade this to debug
logging if it's a permission problem.

(This wasn't an issue before b92f3507
because back then the failures wouldn't be detected at all.)

(cherry picked from commit 35b4e3c1)