A stress test repeatedly resetting the adapter while performing io would
eventually report I/O failures and missing nvme namespaces.

The driver was setting the nvmefc_fcp_req->private pointer to NULL
during the IO completion routine before upcalling done().  If the
transport was also running an abort for that IO, the driver would fail
the abort with message 6140. Failing the abort is not allowed by the
nvme-fc transport, as it mandates that the io must be returned back to
the transport. As that does not happen, the transport controller delete
has an outstanding reference and can't complete teardown.

The NULL-ing of the private pointer should be done only when the io is
considered complete. It's complete when the adapter returns the exchange
with the "exchange busy" flag clear.

Move the NULL'ing of the structure to the done case. This leaves the io
contexts set while it is busy and until the subsequent XRI_ABORTED
completion which returns the exchange is received.
Signed-off-by: Dick Kennedy <dick.kennedy@broadcom.com>
Signed-off-by: James Smart <james.smart@broadcom.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>