nvme: fix srcu locking on error return in nvme_get_ns_from_disk

If we can't get a namespace don't leak the SRCU lock.  nvme_ioctl was
working around this, but nvme_pr_command wasn't handling this properly.
Just do what callers would usually expect.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <keith.busch@intel.com>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>