memif: Process bad descriptors correctly in memif_process_desc

memif: Process bad descriptors correctly in memif_process_desc

When there is a bad descriptor, it may in the beginning, in the middle,
or at the end of the batch if the batch has more than 3 descriptors.
When processing a bad descriptor is encountered in the batch, we need to
rollback n_buffers in memif_process_desc(), or the statement in the same
function
	    memif_add_copy_op (ptd, mb0 + src_off, bytes_to_copy,
                               dst_off, n_buffers - 1);
is wrong because it picks up the wrong buffer_vec_index of the bad
descriptor while parsing a good descriptor immediately following the
bad descriptor. n_buffers was incremented in the beginning of
while (n_left) loop.

The other problem is we should count the number of bad packets and
reduce ptd->n_packets to get the correct number of packets for subsequent
processing in device_input.

The last fix is to check if n_buffers == 0 in device_input and skip
doing any descriptor copy. This case can happen when all the descriptors
are bad in the batch.

Type: fix

Signed-off-by: Steven Luong <sluong@cisco.com>
Change-Id: I28ed1d87236b045657006755747b5750a9a733be