fuse-uring: refactor io-uring header copying from ring

fuse-uring: refactor io-uring header copying from ring

Move header copying from ring logic into a new copy_header_from_ring()
function. This makes the copy_from_user() logic more clear and
centralizes error handling / rate-limited logging.

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>