cifs: Move the 'pid' from the subreq to the req

Move the reference pid from the cifs_io_subrequest struct to the
cifs_io_request struct as it's the same for all subreqs of a particular
request.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Paulo Alcantara <pc@manguebit.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: linux-cifs@vger.kernel.org
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Steve French <stfrench@microsoft.com>