nvme: do not call del_gendisk() on a disk that was never added

nvme: do not call del_gendisk() on a disk that was never added

device_add_disk() is negated by del_gendisk().
alloc_disk_node() is negated by put_disk().

In nvme_alloc_ns(), device_add_disk() is one of the last things being
called in the success case, and only void functions are being called
after this. Therefore this call should not be negated in the error path.

The superfluous call to del_gendisk() leads to the following prints:
[    7.839975] kobject: '(null)' (000000001ff73734): is not initialized, yet kobject_put() is being called.
[    7.840865] WARNING: CPU: 2 PID: 361 at lib/kobject.c:736 kobject_put+0x70/0x120

Fixes: 86984936632c ("nvme: enforce extended LBA format for fabrics metadata")
Signed-off-by: Niklas Cassel <niklas.cassel@wdc.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Max Gurtovoy <maxg@mellanox.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>