eventfd: Rename `struct eventfd` to `struct eventfd_ctx`

[dumbbell]

Summary

This is the name of the structure on Linux. I'm about to expose it through linuxkpi as an opaque type. Therefore it allows to use it directly without further compatibility code in linuxkpi.

This structure is otherwise internal to sys/kern/sys_eventfd.c and not used anywhere else. Thus it is safe to change its name without breaking anything.

This is a requirement to the following patch to drm-kmod:
https://github.com/freebsd/drm-kmod/pull/358

Sponsored by: The FreeBSD Foundation

[emaste]

Hmm, OK. The struct is different and we might end up with a slightly puzzling build failure if things end up cross-threaded, but I suppose it's not really different than the status quo.

[dumbbell]

In D50848#1164156, @emaste wrote:

Hmm, OK. The struct is different and we might end up with a slightly puzzling build failure if things end up cross-threaded, but I suppose it's not really different than the status quo.

What do you mean by "cross-threaded"?

[emaste]

What do you mean by "cross-threaded"?

Sorry, it's a colloquialism. I mean that if we somehow end up including Linuxy and FreeBSD headers in the same context we could have an unclear view of what struct eventfd_ctx looks like. But yeah, if it's only ever used as an opaque pointer in Linux code it won't be an issue.