debugfs: fix debugfs_real_fops() build error

Some drivers use debugfs_real_fops() even when CONFIG_DEBUG_FS is disabled,
which now leads to a build error:

In file included from include/linux/list.h:9:0,
                 from include/linux/wait.h:7,
                 from include/linux/wait_bit.h:8,
                 from include/linux/fs.h:6,
                 from drivers/net/wireless/broadcom/b43legacy/debugfs.c:26:
drivers/net/wireless/broadcom/b43legacy/debugfs.c: In function 'b43legacy_debugfs_read':
drivers/net/wireless/broadcom/b43legacy/debugfs.c:224:23: error: implicit declaration of function 'debugfs_real_fops'; did you mean 'debugfs_create_bool'? [-Werror=implicit-function-declaration]

My first impulse was to add another 'static inline' dummy function
returning NULL for it, which would work fine. However, most callers
feed the pointer into container_of(), so it seems a little dangerous
here. Since all the callers are inside of a read/write file operation
that gets eliminated in this configuration, so having an 'extern'
declaration seems better here. If it ever gets used in a dangerous
way, that will now result in a link error.

Fixes: 7c8d469877 ("debugfs: add support for more elaborate ->d_fsdata")
Cc: Jakub Kicinski <jakub.kicinski@netronome.com>
Cc: Simon Horman <simon.horman@netronome.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/debugfs.h

@@ -216,6 +216,8 @@ static inline void debugfs_remove(struct dentry *dentry)
 static inline void debugfs_remove_recursive(struct dentry *dentry)
 { }
 
+const struct file_operations *debugfs_real_fops(const struct file *filp);
+
 static inline int debugfs_file_get(struct dentry *dentry)
 {
 	return 0;