CLEANUP: net_helper: Do not negate the result of unlikely

CLEANUP: net_helper: Do not negate the result of unlikely

This patch turns the double negation of 'not unlikely' into 'likely'
and then turns the negation of 'not smaller' into 'greater or equal'
in an attempt to improve readability of the condition.

[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 ]