rxe: fix completion queue consumer index overrun

[dragonJACson]

Fix a bug in the CQ polling sequence where the consumer index can incorrectly advance beyond available completions.

Consider this polling sequence:

    ibv_start_poll();
    ibv_next_poll();
    ibv_end_poll();
    ibv_start_poll();

With one completion in the queue, the indices would be:

1. After ibv_start_poll() (reading first CQE):

     P 
┌──┬─┴┬──┬──┬──┐
└─┬┴──┴──┴──┴──┘ 
  C

2. After ibv_next_poll() (returns ENOENT):

     P 
┌──┬─┴┬──┬──┬──┐
└──┴─┬┴──┴──┴──┘
     C

3. After ibv_end_poll():

     P 
┌──┬─┴┬──┬──┬──┐ 
└──┴──┴─┬┴──┴──┘ 
        C

The issue occurs because ibv_end_poll() advances the consumer index even after ibv_next_poll() returns ENOENT. This causes the consumer index to move beyond the producer index, leading to:

• False indication of available completions
• Reading of uninitialized completion entries

Fix this by checking for available completions before advancing the consumer index in ibv_next_poll().

Note: According to the man page, ibv_end_poll() must be called even when ibv_next_poll() returns ENOENT, but consumer index should only be advanced once in this case.