RDMA/core: Fix for parsing netlink string attribute

The string iwpm_ulib_name is recorded in a nlmsg as a netlink attribute.
Without this fix parsing of the nlmsg by the userspace port mapper service fails
because of unknown attribute length, causing the port mapper service not to
register the client, which has sent the nlmsg.

Signed-off-by: Tatyana Nikolova <tatyana.e.nikolova@intel.com>
Cc: <stable@vger.kernel.org> #v3.16
Reviewed-By: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>