parse nvme connect Linux error codes properly

[Engel, Amit]

Hello,

we have encountered a wrong nvme/host/fabrics.c behavior as part of connect() failure.

Current nvmf_log_connect_error() code is assuming that 'int errval' is an unsigned nvme error, so it clears bit DNR=0x4000
The code should check if the error is negative (Linux error code) first.
If so, need to have a suitable log and return.

I can provide a simple fix for your review if you have no comments to the above analysis 

Thanks,
Amit Engel