Use cpuset_getaffinity instead of sysconf(_SC_NPROCESSORS_ONLN)

[asomers]

Describe the feature would like to see added to OpenZFS

Several functions in libzfs and the zfs commands construct thread pools sized according to the number of CPUs in the system. They determine that via the sysconf(_SC_NPROCESSORS_ONLN) function. But that reports that total number of CPUs in the system. It would be more appropriate to use the number of CPUs available to the currrent process instead. That can be determined via cpuset_getaffinity on FreeBSD or sched_getaffinity on Linux.

How will this feature improve OpenZFS?

It will prevent thread pools from being oversized wherever the system administrator has configured cpusets.

Additional context

https://man.freebsd.org/cgi/man.cgi?query=cpuset_getaffinity&sektion=2&n=1
https://linux.die.net/man/2/sched_getaffinity
https://github.com/freebsd/freebsd-src/blob/0612538e3ac93c1884f595a72609c078aefbcc28/bin/nproc/nproc.c#L118