RDMA/rdmavt: 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.

Signed-off-by: Shen Lichuan <shenlichuan@vivo.com>
Link: https://patch.msgid.link/20240828082720.33231-1-shenlichuan@vivo.com


Signed-off-by: Leon Romanovsky <leon@kernel.org>