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: aff03707 ("sched/psi: use kernfs polling functions for PSI trigger polling")
Fixes: 147e1a97 ("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.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/kernfs.h

@@ -550,6 +550,10 @@ static inline int kernfs_setattr(struct kernfs_node *kn,
 				 const struct iattr *iattr)
 { return -ENOSYS; }
 
+static inline __poll_t kernfs_generic_poll(struct kernfs_open_file *of,
+					   struct poll_table_struct *pt)
+{ return -ENOSYS; }
+
 static inline void kernfs_notify(struct kernfs_node *kn) { }
 
 static inline int kernfs_xattr_get(struct kernfs_node *kn, const char *name,