nvme: code cleanup nvme_identify_ns_desc()

The function nvme_identify_ns_desc() has 3 levels of nesting which make
error message to exceeded > 80 char per line which is not aligned with
the kernel code standards and rest of the NVMe subsystem code.

Add a helper function to move the processing of the log when the
command is successful by reducing the nesting and keeping the
code < 80 char per line.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>