RDMA/iwpm: Properly mark end of NL messages

Commit 1a1c116f removes nlmsg_len calculation in
ibnl_put_attr causing netlink messages to be rejected due
to incorrect length.

Add nlmsg_end after all attributes are appended to calculate
the nlmsg_len.

Fixes: 1a1c116f ("RDMA/netlink: Simplify the put_msg and put_attr")
Signed-off-by: Shiraz Saleem <shiraz.saleem@intel.com>
Signed-off-by: Tatyana Nikolova <tatyana.e.nikolova@intel.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>