fuse-uring: use enum types for header copying

fuse-uring: use enum types for header copying

Use enum types to identify which part of the header needs to be copied.
This improves the interface and will simplify both kernel-space and
user-space header addresses copying when buffer rings are added.

Reviewed-by: Bernd Schubert <bschubert@ddn.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Baokun Li <libaokun@linux.alibaba.com>
Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>