MEDIUM: connection: panic when calling FD-specific functions on FD-less conns

MEDIUM: connection: panic when calling FD-specific functions on FD-less conns

Certain functions cannot be called on an FD-less conn because they are
normally called as part of the protocol-specific setup/teardown sequence.
Better place a few BUG_ON() to make sure none of them is called in other
situations. If any of them would trigger in ambiguous conditions, it would
always be possible to replace it with an error.