BUG/MINOR: check: do not inspect errno if conn FDLESS

BUG/MINOR: check: do not inspect errno if conn FDLESS

On check failure, conn errno is inspected via
retrieve_errno_from_socket(), which relies on the connection FD. When
using a CO_FL_FDLESS connection, this triggers a BUG_ON().

This causes crashes for QUIC backend checks, available since 3.3. To fix
this, simply skip call to retrieve_errno_from_socket() when CO_FL_FDLESS
is set.

This must be backported up to 3.3.