fix ambiguous if statements

fix ambiguous if statements

the funciton nflog_bind_pf(...) returns an integer smaller 0 on a
failure, we negated that which results in 1 if no failure and 0 if
there was a failure.
This is ambiguous and as no parenthesis are set the GCC 6 warning
"logical-not-parentheses" gets triggered.

Use a simple
    nflog_bind_pf(...) < 0
check instead.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>