RDMA/cxgb4: Use refcount_t instead of atomic_t for reference counting

The refcount_t API will WARN on underflow and overflow of a reference
counter, and avoid use-after-free risks.

Link: https://lore.kernel.org/r/1622194663-2383-12-git-send-email-liweihang@huawei.com


Cc: Potnuri Bharat Teja <bharat@chelsio.com>
Signed-off-by: Weihang Li <liweihang@huawei.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>