RDMA/rxe: Simplify the error handling code in rxe_create_ah()

This patch not only simplifies the error handling code in rxe_create_ah()
but also removes the dead code that was left behind by commit 47ec3866
("RDMA: Convert drivers to use sgid_attr instead of sgid_index").

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Reviewed-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>