Merge branch 'mpls-more-labels'

David Ahern says:

====================
net: mpls: Allow users to configure more labels per route

Increase the maximum number of new labels for MPLS routes from 2 to 30.

To keep memory consumption in check, the labels array is moved to the end
of mpls_nh and mpls_iptunnel_encap structs as a 0-sized array. Allocations
use the maximum number of labels across all nexthops in a route for LSR
and the number of labels configured for LWT.

The mpls_route layout is changed to:

   +----------------------+
   | mpls_route           |
   +----------------------+
   | mpls_nh 0            |
   +----------------------+
   | alignment padding    |   4 bytes for odd number of labels; 0 for even
   +----------------------+
   | via[rt_max_alen] 0   |
   +----------------------+
   | alignment padding    |   via's aligned on sizeof(unsigned long)
   +----------------------+
   | ...                  |

Meaning the via follows its mpls_nh providing better locality as the
number of labels increases. UDP_RR tests with namespaces shows no impact
to a modest performance increase with this layout for 1 or 2 labels and
1 or 2 nexthops.

mpls_route allocation size is limited to 4096 bytes allowing on the
order of 30 nexthops with 30 labels (or more nexthops with fewer
labels). LWT encap shares same maximum number of labels as mpls routing.

v3
- initialize n_labels to 0 in case RTA_NEWDST is not defined; detected
  by the kbuild test robot

v2
- updates per Eric's comments
  + added patch to ensure all reads of rt_nhn_alive and nh_flags in
    the packet path use READ_ONCE and all writes via event handlers
    use WRITE_ONCE

  + limit mpls_route size to 4096 (PAGE_SIZE for most arch)

  + mostly killed use of MAX_NEW_LABELS; it exists only for common
    limit between lwt and routing paths
====================

Signed-off-by: David S. Miller <davem@davemloft.net>