port: fix unaligned access to metadata

port: fix unaligned access to metadata

Fix RTE_MBUF_METADATA macros to allow for unaligned accesses to
meta-data fields.
Forcing aligned accesses is not really required, so this is removing an
unneeded constraint.
This issue was met during testing of the new version of the ip_pipeline
application. There is no performance impact.
This change has no ABI impact, as the previous code that uses aligned
accesses continues to run without any issues.

Signed-off-by: Daniel Mrzyglod <danielx.t.mrzyglod@intel.com>
Acked-by: Cristian Dumitrescu <cristian.dumitrescu@intel.com>