CLEANUP: conn: Do not pass a pointer to likely

CLEANUP: conn: Do not pass a pointer to likely

Move the `!` inside the likely and negate it to unlikely.

The previous version should not have caused issues, because it is converted
to a boolean / integral value before being passed to __builtin_expect(), but
it's certainly unusual.

[wt: this was not a bug but purposely written like this to improve code
 generation on older compilers but not needed anymore as described here:
 https://www.mail-archive.com/haproxy@formilux.org/msg36392.html ]