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>