nvme: use define instead of magic value for identify size

NVME_IDENTIFY_DATA_SIZE was added to linux/nvme.h by following commit.
  commit 0add5e8e ("nvmet: use NVME_IDENTIFY_DATA_SIZE")

Make it use NVME_IDENTIFY_DATA_SIZE define instead of magic value
0x1000 in case of identify data size.

Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Minwoo Im <minwoo.im.dev@gmail.com>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Jens Axboe <axboe@kernel.dk>