Revert "cpuset: increase userland maximum size to 1024"

This reverts commit 76887e84be975698b14699d7d0dfb157d73e9990.

struct vm_exit currently requires that cpuset_t be identical in userland
and kernel. This will be recommitted after these are decoupled.

PR: 271330, 269572
Reported by: corvink
Sponsored by: The FreeBSD Foundation