kernfs: add struct dentry declaration in kernfs.h

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 <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>