rdma: fix chained buffer tx path in dv mode

rdma: fix chained buffer tx path in dv mode

When switching to the direct verb chain buffer tx path, we must account
for all remaining packets, including the packets that would wrapped
around.
Previously we were using the 'n' counter but ignoring the 'n_wrap'
counter: if some packets would have wrapped around in the default path,
it would be ignored by the chained buffer tx path.
Compute the correct number of remaining packets based on the old and
current txq tail instead.
Also simplify the chained tx function parameters.

Type: fix

Change-Id: If12b41a8f143fda80290342e2904792f7501c559
Signed-off-by: Benoît Ganne <bganne@cisco.com>