mpipe: fix crash when testpmd is quit under load

mpipe: fix crash when testpmd is quit under load

Fixes: the hung/crash issue when quitting testpmd under high
traffic rate. The following issue were found and fixed.
1. edesc->size is not initialized properly in mpipe_do_xmit() and could
   cause buffer leak or corruption when HW buffer return is used.
2. Check the 'idesc.be' error bit in mpipe_recv_flush() to make sure
   buffer is valid before releasing it. This is to avoid issues when
   running out of buffers.
3. priv->rx_buffers counter is not accurate when HW buffer return is
   used. Remove this counter to simplify the code.

Signed-off-by: Liming Sun <lsun@ezchip.com>
Acked-by: Zhigang Lu <zlu@ezchip.com>