IB/ipoib: Get rid of the tx_outstanding variable in all modes

The first step toward using NAPI in the UD/TX flow is to separate
between two flows, the NAPI and the xmit, meaning no use of shared
variables between both flows.

This patch takes out the tx_outstanding variable that was used in both
flows and instead the driver uses the 2 cyclic ring variables: tx_head
and tx_tail, tx_head used in the xmit flow and tx_tail in the NAPI flow.

Cc: Kamal Heib <kamalh@mellanox.com>
Signed-off-by: Erez Shitrit <erezsh@mellanox.com>
Reviewed-by: Alex Vesker <valex@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>