pq_update() can only be called in two places: from the completion
function when the complete (npkts) sequence of packets has been
submitted and processed, or from setup function if a subset of the
packets were submitted (i.e. the error path).

Currently both paths can call pq_update() if an error occurrs.  This
race will cause the n_req value to go negative, hanging file_close(),
or cause a crash by freeing the txlist more than once.

Several variables are used to determine SDMA send state.  Most of
these are unnecessary, and have code inspectible races between the
setup function and the completion function, in both the send path and
the error path.

The request 'status' value can be set by the setup or by the
completion function.  This is code inspectibly racy.  Since the status
is not needed in the completion code or by the caller it has been
removed.

The request 'done' value races between usage by the setup and the
completion function.  The completion function does not need this.
When the number of processed packets matches npkts, it is done.

The 'has_error' value races between usage of the setup and the
completion function.  This can cause incorrect error handling and leave
the n_req in an incorrect value (i.e. negative).

Simplify the code by removing all of the unneeded state checks and
variables.

Clean up iovs node when it is freed.

Eliminate race conditions in the error path:

If all packets are submitted, the completion handler will set the
completion status correctly (ok or aborted).

If all packets are not submitted, the caller must wait until the
submitted packets have completed, and then set the completion status.

These two change eliminate the race condition in the error path.
Reviewed-by: Mitko Haralanov <mitko.haralanov@intel.com>
Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>