kernfs: add stub helper for kernfs_generic_poll()

kernfs: add stub helper for kernfs_generic_poll()

In some randconfig builds, kernfs ends up being disabled, so there is no prototype
for kernfs_generic_poll()

In file included from kernel/sched/build_utility.c:97:
kernel/sched/psi.c:1479:3: error: implicit declaration of function 'kernfs_generic_poll' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
                kernfs_generic_poll(t->of, wait);
                ^

Add a stub helper for it, as we have it for other kernfs functions.

Fixes: aff037078ecae ("sched/psi: use kernfs polling functions for PSI trigger polling")
Fixes: 147e1a97c4a0b ("fs: kernfs: add poll file operation")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Chengming Zhou <zhouchengming@bytedance.com>
Link: https://lore.kernel.org/r/20230724121823.1357562-1-arnd@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>