eventpoll: fix sphinx documentation build warning

eventpoll: fix sphinx documentation build warning

Sphinx complains that ep_get_upwards_depth_proc() has a kerneldoc-style
comment without documenting its parameters.
This is an internal function that was not meant to show up in kernel
documentation, so fix the warning by changing the comment to a
non-kerneldoc one.

Fixes: 22bacca48a17 ("epoll: prevent creating circular epoll structures")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/r/20250717173655.10ecdce6@canb.auug.org.au
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202507171958.aMcW08Cn-lkp@intel.com/
Signed-off-by: Jann Horn <jannh@google.com>
Link: https://lore.kernel.org/20250721-epoll-sphinx-fix-v1-1-b695c92bf009@google.com
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>