Fix regression introduced by commit 8088c05c50

Regression:
CIDR matching code now returns an incorrect result when dealing
with netmasks that are not a multiple of 8 bits.

Bug:
'-1 << x' is not the same as '(1 << x) - 1', but rather the
inverse of the latter.

Fix:
Invert the result.

Reported-by: shadowninja@minetest.net