packets: Fix misaligned data accesses for MPLS and SCTP fields.

The other 32-bit data fields in protocol headers were already using
ovs_16aligned_be32, but MPLS and SCTP had been overlooked.  This fixes
the failure of test 681 seen here:
https://buildd.debian.org/status/fetch.php?pkg=openvswitch&arch=sparc&ver=2.1.0%2Bgit20140325-1&stamp=1396438624

Signed-off-by: Ben Pfaff <blp@nicira.com>