nvmf: keep track of nvmet connect error status

To let the host know what happends to the connection establishment,
adjust the behavior of nvmf_log_connect_error to make more connect
specifig error codes human-readble.

Signed-off-by: Guan Junxiong <guanjunxiong@huawei.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>