sys-cluster/ceph currently does not have a use flag to enable RDMA.
I've opened a pull request to resolve this: https://github.com/gentoo/gentoo/pull/25244
It appears that enabling the WITH_RDMA options introduces new dependencies. https://github.com/ceph/ceph/blob/master/CMakeLists.txt#L143 Your pull request does not update DEPEND/RDEPEND.