MAINTAINERS: add Stefan Hajnoczi as block/nvme.c maintainer

MAINTAINERS: add Stefan Hajnoczi as block/nvme.c maintainer

Development of the userspace NVMe block driver picked up again recently.
After talking with Fam I am stepping up as block/nvme.c maintainer.
Patches will be merged through my 'block' tree.

Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Klaus Jensen <k.jensen@samsung.com>
Cc: Fam Zheng <fam@euphon.net>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Acked-by: Kevin Wolf <kwolf@redhat.com>
Acked-by: Klaus Jensen <k.jensen@samsung.com>
Acked-by: Fam Zheng <fam@euphon.net>
Message-id: 20200907111632.90499-1-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>