IPoIB: Refactor completion handling

Split up ipoib_ib_handle_wc() into ipoib_ib_handle_rx_wc() and
ipoib_ib_handle_tx_wc() to make the code easier to read.  This will
also help implement NAPI in the future.

Signed-off-by: Roland Dreier <rolandd@cisco.com>