IB/umad: Do not check status of nonseekable_open()

As the comment block of nonseekable_open() describes, nonseekable_open()
can never fail. Several places in kernel depend on this behavior.
Therefore, simplify the umad module to depend on this basic kernel
functionality.

Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>