fuse-uring: use named constants for io-uring iovec indices

fuse-uring: use named constants for io-uring iovec indices

Replace magic indices 0 and 1 for the iovec array with named constants
FUSE_URING_IOV_HEADERS and FUSE_URING_IOV_PAYLOAD. This makes the usages
self-documenting and prepares for buffer ring support which will also
reference these iovec slots by index.

Reviewed-by: Bernd Schubert <bernd@bsbernd.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>