nvme: fix inconsistent RCU list manipulation in nvme_ns_add_to_ctrl_list()

When inserting a namespace into the controller's namespace list, the
function uses list_add_rcu() when the namespace is inserted in the middle
of the list, but falls back to a regular list_add() when adding at the
head of the list.

This inconsistency could lead to race conditions during concurrent
access, as users might observe a partially updated list. Fix this by
consistently using list_add_rcu() in both code paths to ensure proper
RCU protection throughout the entire function.

Fixes: be647e2c ("nvme: use srcu for iterating namespace list")
Signed-off-by: Zheng Qixing <zhengqixing@huawei.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>