x/sys/unix: solaris EventPort panic when processing events after Close

[nshalman]

While stress testing fsnotify/fsnotify#371 I ran into a panic that I tracked down to
one thread calling Close() while another thread was still trying to process one or more events
it had already gotten back from the OS.

A CR will follow shortly.

[gopherbot]

Change https://go.dev/cl/422338 mentions this issue: unix: Fix EventPort panic after Close

[nshalman]

@mknyszek you can assign this one to me as well.