[NETFILTER]: xt_hashlimit: fix mask calculation

Shifts larger than the data type are undefined, don't try to shift
an u32 by 32. Also remove some special-casing of bitmasks divisible
by 32.

Based on patch by Jan Engelhardt <jengelh@computergmbh.de>.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>