docs: futex: Fix kernel-doc references after code split-up preparation

In upstream commit 77e52ae3 ("futex: Move to kernel/futex/") the
futex code from kernel/futex.c was moved into kernel/futex/core.c in
preparation of the split-up of the implementation in various files.

Point kernel-doc references to the new files as otherwise the
documentation shows errors on build:

    [...]
    Error: Cannot open file ./kernel/futex.c
    Error: Cannot open file ./kernel/futex.c
    [...]
    WARNING: kernel-doc './scripts/kernel-doc -rst -enable-lineno -sphinx-version 3.4.3 -internal ./kernel/futex.c' failed with return code 2

There is no direct upstream commit for this change. It is made in
analogy to commit bc67f1c4 ("docs: futex: Fix kernel-doc
references") applied as consequence of the restructuring of the futex
code.

Fixes: 77e52ae3

 ("futex: Move to kernel/futex/")
Signed-off-by: Salvatore Bonaccorso <carnil@debian.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>