File operations

[Wedson Almeida Filho <wedsonaf@google.com>]

Hello,

I'm looking to implement a FileOperations trait for a driver and am
finding it cumbersome. For example, to implement read, one needs to
declare the following (copied from example):

impl file_operations::FileOperations for CycleFile {
    const READ: file_operations::ReadFn<Self> = Some(
        |_this: &Self,
         _file: &file_operations::File,
         _buf: &mut user_ptr::UserSlicePtrWriter,
         _offset: u64|
         -> KernelResult<()> {
         ...
       },
    );
}

I couldn't find a way to simplify this to just implementing the
desired functions, but I can get it down to implementing just the
desired functions plus one annotation for the impl block, for example
(note the [#build_fileops] annotation at the top):

[#build_fileops]
impl file_operations::FileOperations for CycleFile {
    fn read(
        &self,
        _file: &file_operations::File,
        _buf: &mut user_ptr::UserSlicePtrWriter,
        _offset: u64,
    ) -> KernelResult() {
        ...
    }
}

How do you all feel about this? I think it is an improvement but would
like to hear what you think before spending more time on this.

Thanks,
-Wedson