linuxkpi: Add eventfd_*()

Add <linux/eventfd.h> and expose the eventfd_*() API. This is used by
DRM drivers for some time, but the code was commented out so far.

Note that Linux uses struct eventfd_ctx, but FreeBSD defines `struct
eventfd`. We define eventfd_ctx as a synonym to eventfd.

Reviewed by: christos, markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D50853