eventfd: fold eventfd_ctx_read() into eventfd_read()
eventfd_ctx_read() is not used outside of eventfd.c, so unexport it and
fold it into eventfd_read(). This slightly simplifies the code and
makes it more analogous to eventfd_write().
(eventfd_ctx_read() was apparently added years ago for KVM irqfd's, but
was never used.)
Signed-off-by: Eric Biggers <ebiggers@google.com> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>