kernfs: add struct dentry declaration in kernfs.h

Hello, Greg.

Two misc fixes for kernfs.

Thanks.
------- 8< -------
struct dentry is used in kernfs.h but its declaration was missing,
leading to compilation errors unless its declaration gets pulled in in
some other way. Add the declaration.

Signed-off-by: Tejun Heo <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

include/linux/kernfs.h

···
 #include <linux/completion.h>
 
 struct file;
+struct dentry;
 struct iattr;
 struct seq_file;
 struct vm_area_struct;