tree: Move global device info to a single struct

tree: Move global device info to a single struct

Currently, we have a couple of globals that refer to the current NVMe
device: the device name, and the stat buffer:

    static struct stat nvme_stat;
    const char *devicename;

This change moves those two globals into a single global struct:

    struct nvme_dev {
            struct stat stat;
            const char *name;
    };

    extern struct nvme_dev *nvme_dev;

This will make it easier to constrain the scope of these globals in a
later change.

Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>