RDMA/cxgb4: Save the correct map length for fast_reg_page_lists

We cannot save the mapped length using the rdma max_page_list_len field
of the ib_fast_reg_page_list struct because the core code uses it.  This
results in an incorrect unmap of the page list in c4iw_free_fastreg_pbl().

I found this with dma mapping debugging enabled in the kernel.  The
fix is to save the length in the c4iw_fr_page_list struct.

Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>

drivers/infiniband/hw/cxgb4/iw_cxgb4.h

@@ -369,6 +369,7 @@ struct c4iw_fr_page_list {
 	DEFINE_DMA_UNMAP_ADDR(mapping);
 	dma_addr_t dma_addr;
 	struct c4iw_dev *dev;
+	int pll_len;
 };
 
 static inline struct c4iw_fr_page_list *to_c4iw_fr_page_list(