nvme: add a sparse annotation to nvme_ns_head_ctrl_ioctl

Add the __releases annotation to tell sparse that nvme_ns_head_ctrl_ioctl
is expected to unlock head->srcu.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>