rbd: avoid get_callback_adapter() for tcp_stream::async_connect()

rbd: avoid get_callback_adapter() for tcp_stream::async_connect()

works around a compilation failure in c++20 (with gcc 11.2 and boost
1.76) when choosing between two overloads of
`boost::beast::tcp_stream::async_connect()`

`get_callback_adapter()` returns a variadic lambda that matches the
concept for both overloads (`completion_token_for<ConnectHandler>`
and `completion_token_for<RangeConnectHandler>`), but compilation of
the wrapped lambda fails for the `ConnectHandler` overload because it
expects two arguments instead of one

instead of using `get_callback_adapter()` to convert the first argument
from `boost::system::error_code` to `int` for the wrapped lambda, do
this in the lambda itself

Fixes: https://tracker.ceph.com/issues/54303
Signed-off-by: Casey Bodley <cbodley@redhat.com>