IB/mlx5: Fix initializing CQ fragments buffer

IB/mlx5: Fix initializing CQ fragments buffer

The function init_cq_frag_buf() can be called to initialize the current CQ
fragments buffer cq->buf, or the temporary cq->resize_buf that is filled
during CQ resize operation.

However, the offending commit started to use function get_cqe() for
getting the CQEs, the issue with this change is that get_cqe() always
returns CQEs from cq->buf, which leads us to initialize the wrong buffer,
and in case of enlarging the CQ we try to access elements beyond the size
of the current cq->buf and eventually hit a kernel panic.

 [exception RIP: init_cq_frag_buf+103]
  [ffff9f799ddcbcd8] mlx5_ib_resize_cq at ffffffffc0835d60 [mlx5_ib]
  [ffff9f799ddcbdb0] ib_resize_cq at ffffffffc05270df [ib_core]
  [ffff9f799ddcbdc0] llt_rdma_setup_qp at ffffffffc0a6a712 [llt]
  [ffff9f799ddcbe10] llt_rdma_cc_event_action at ffffffffc0a6b411 [llt]
  [ffff9f799ddcbe98] llt_rdma_client_conn_thread at ffffffffc0a6bb75 [llt]
  [ffff9f799ddcbec8] kthread at ffffffffa66c5da1
  [ffff9f799ddcbf50] ret_from_fork_nospec_begin at ffffffffa6d95ddd

Fix it by getting the needed CQE by calling mlx5_frag_buf_get_wqe() that
takes the correct source buffer as a parameter.

Fixes: 388ca8be0037 ("IB/mlx5: Implement fragmented completion queue (CQ)")
Link: https://lore.kernel.org/r/90a0e8c924093cfa50a482880ad7e7edb73dc19a.1623309971.git.leonro@nvidia.com
Signed-off-by: Alaa Hleihel <alaa@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>