RDMA/core: Convert to use ERR_CAST()

As opposed to open-code, using the ERR_CAST macro clearly indicates that
this is a pointer to an error value and a type conversion was performed.

Link: https://patch.msgid.link/r/20250401210840146_IyrV3zlejzz3eAnDmMSB@zte.com.cn


Signed-off-by: Li Haoran <li.haoran7@zte.com.cn>
Signed-off-by: Shao Mingyin <shao.mingyin@zte.com.cn>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>