ipv6: Fix fragment id assignment on LE arches.

Recent commit:
0508c07f
Author: Vlad Yasevich <vyasevich@gmail.com>
Date:   Tue Feb 3 16:36:15 2015 -0500

    ipv6: Select fragment id during UFO segmentation if not set.

Introduced a bug on LE in how ipv6 fragment id is assigned.
This was cought by nightly sparce check:

Resolve the following sparce error:
 net/ipv6/output_core.c:57:38: sparse: incorrect type in assignment
 (different base types)
   net/ipv6/output_core.c:57:38:    expected restricted __be32
[usertype] ip6_frag_id
   net/ipv6/output_core.c:57:38:    got unsigned int [unsigned]
[assigned] [usertype] id

Fixes: 0508c07f (ipv6: Select fragment id during UFO segmentation if not set.)
Signed-off-by: Vladislav Yasevich <vyasevic@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>