pf: Don't convert arc4random() to host byte order

There's no need to convert values returned by arc4random to the network
byte order. Spotted by Gleb Smirnoff (glebius@FreeBSD.org), thanks!

ok tedu

Obtained from: OpenBSD, mikeb <mikeb@openbsd.org>, 367b0410d3
Sponsored by: Rubicon Communications, LLC ("Netgate")