nvme: Don't seg fault if given device is not char/block device

nvme: Don't seg fault if given device is not char/block device

If the given device name is not char/block device then in
open_dev_direct() set errno to ENODEV and err to -1 before
returning to the dev_fd(). This would then ensure that in
case of error, dev_fd() returns the corerct negative error
code back to its callers. So now the callers of dev_fd()
would handle the error appropriately instead of try
accessing the nvme_dev which would be NULL.

Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
Signed-off-by: Daniel Wagner <dwagner@suse.de>