Initialize field owner in file_operations

Initialized the field `owner` in struct `file_operations`. It's used
to increment the module ref count when a file is opened, to prevent the
module from being unloaded.

Fixes the issue #214

Signed-off-by: Roni Nevalainen <kitten@kittenz.dev>

Author: Roni Nevalainen

rust/kernel/file_operations.rs

@@ -315,7 +315,7 @@ impl<A: FileOpenAdapter, T: FileOpener<A::Arg>> FileOperationsVtable<A, T> {
             None
         },
         mmap_supported_flags: 0,
-        owner: ptr::null_mut(),
+        owner: crate::ThisModule as *mut bindings::module,
         poll: if T::TO_USE.poll {
             Some(poll_callback::<T>)
         } else {