cxgb4: add support for mirror Rxqs

When mirror VI is enabled, allocate the mirror Rxqs and setup the
mirror VI RSS table. The mirror Rxqs are allocated/freed when
the mirror VI is created/destroyed or when underlying port is
brought up/down, respectively.

v3:
- Replace mirror VI refcount_t with normal u32 variable.

v2:
- Use mutex to protect all mirror VI data, instead of just
  mirror Rxqs.
- Remove the un-needed mirror Rxq mutex.

Signed-off-by: Rahul Lakkireddy <rahul.lakkireddy@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>