wireguard: ratelimiter: use kvcalloc() instead of kvzalloc()

Use 2-factor argument form kvcalloc() instead of kvzalloc().

Link: https://github.com/KSPP/linux/issues/162
Fixes: e7096c13

 ("net: WireGuard secure network tunnel")
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
[Jason: Gustavo's link above is for KSPP, but this isn't actually a
 security fix, as table_size is bounded to 8192 anyway, and gcc realizes
 this, so the codegen comes out to be about the same.]
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>