nvme: Treat discovery subsystems as unique subsystems

Current code matches subnqn and collapses all controllers to the
same subnqn to a single subsystem structure. This is good for
recognizing multiple controllers for the same subsystem. But with
the well-known discovery subnqn, the subsystems aren't truly the
same subsystem. As such, subsystem specific rules, such as no
overlap of controller id, do not apply. With today's behavior, the
check for overlap of controller id can fail, preventing the new
discovery controller from being created.

When searching for like subsystem nqn, exclude the discovery nqn
from matching. This will result in each discovery controller being
attached to a unique subsystem structure.
Signed-off-by: James Smart <jsmart2021@gmail.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Max Gurtovoy <maxg@mellanox.com>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>

drivers/nvme/host/core.c

@@ -2374,6 +2374,17 @@ static struct nvme_subsystem *__nvme_find_get_subsystem(const char *subsysnqn)
 
 	lockdep_assert_held(&nvme_subsystems_lock);
 
+	/*
+	 * Fail matches for discovery subsystems. This results
+	 * in each discovery controller bound to a unique subsystem.
+	 * This avoids issues with validating controller values
+	 * that can only be true when there is a single unique subsystem.
+	 * There may be multiple and completely independent entities
+	 * that provide discovery controllers.
+	 */
+	if (!strcmp(subsysnqn, NVME_DISC_SUBSYS_NAME))
+		return NULL;
+
 	list_for_each_entry(subsys, &nvme_subsystems, entry) {
 		if (strcmp(subsys->subnqn, subsysnqn))
 			continue;