datapath: Use ipv4_dst_hoplimit() instead of direct access.

datapath: Use ipv4_dst_hoplimit() instead of direct access.

In 2.6.38 dst metrics became shared and use copy-on-write instead
of using private versions.  This means that it is no longer permissible
to directly access the metric and we need to use the helper function
instead.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>