RDMA/usnic: Do not use ucontext->tgid

Update this driver to match the code it copies from umem.c which no longer
uses tgid.

Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>