RDMA/rdmavt: Fix error code in rvt_create_qp()

Return negative -ENOMEM instead of positive ENOMEM.  Returning a postive
value will cause an Oops because it becomes an ERR_PTR() in the
create_qp() function.

Fixes: 514aee66

 ("RDMA: Globally allocate and release QP memory")
Link: https://lore.kernel.org/r/20211013080645.GD6010@kili
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Dennis Dalessandro <dennis.dalessandro@cornelisnetworks.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>