iw_cxgb4: initialize ib_mr fields for user mrs

Some of the struct ib_mr fields weren't getting initialized.  This was
benign, but will cause problems when dumping the mr resource via
nldev/restrack.

Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>