MFC r321605:

As in r315225, discard 3072 bytes of RC4 bytestream instead of 1024.
(This implementation of arc4rand(9) is used by the userland ipftest
utility as it approximates ipfilter kernelspace in userspace.)

PR: 217920
Submitted by: codarren@hackers.mu
Reviewed by: emaste, cem
Approved by: so (implicit, in r315225)
Differential Revision: D11747
Patterned after: r315225