nvme: fix potential memory leak in option parsing

When specifying same string type option several times,
current option parsing may cause memory leak. Hence,
call kfree for previous one in this case.

Signed-off-by: Chengguang Xu <cgxu519@gmx.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>