msg/mds.h: use PACKED and add 'base' field
msg/mds.h: use PACKED and add 'base' field

Make sizeof work correctly on messages destined for the MDS.

Signed-off-by: Colin McCabe <3142ce8a0bdba4a38073365893e6b74f72d9c512@alumni.cmu.edu>

cmccabe/redfish,cmccabe/redfish,cmccabe/redfish,cmccabe/redfish,cmccabe/redfish