eventfd: Add eventfd_signal()

The eventfd_signal() function is the equivalent to a write to an
eventfd file descriptor: it bumps the internal counter and wakes up
processes waiting for it.

eventfd_signal() is meant to be used by kernel drivers. DRM drivers
will call it through linuxkpi.

Reviewed by: markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D50850