nvme-core: remove extra variable

In nvme_validate_ns() the exra variable ctrl is used only twice.
Using ns->ctrl directly still maintains the redability and original
length of the lines in the code. Get rid of the extra variable ctrl &
use ns->ctrl directly.

Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>