vader: ensure fast box data is aligned on 4-byte boundaries

This commit fixes a bus error on Solaris/Sparc.

Closes trac:4111

cmr=v1.7.5:ticket=trac:4053

This commit was SVN r30288.

The following Trac tickets were found above:
  Ticket 4053 --> https://svn.open-mpi.org/trac/ompi/ticket/4053
  Ticket 4111 --> https://svn.open-mpi.org/trac/ompi/ticket/4111

ompi/mca/btl/vader/btl_vader_fbox.h

@@ -26,7 +26,7 @@
  * critical path */
 #define MCA_BTL_VADER_LAST_FBOX 63
 /* two bytes are reserved for tag and size (update if the header is modified) */
-#define MCA_BTL_VADER_FBOX_HDR_SIZE 2
+#define MCA_BTL_VADER_FBOX_HDR_SIZE 4
 #define MCA_BTL_VADER_FBOX_MAX_SIZE (MCA_BTL_VADER_FBOX_SIZE - MCA_BTL_VADER_FBOX_HDR_SIZE)
 /* total size of all the fast boxes assigned to a particular peer */
 #define MCA_BTL_VADER_FBOX_PEER_SIZE (MCA_BTL_VADER_FBOX_SIZE * (MCA_BTL_VADER_LAST_FBOX + 1))
@@ -34,10 +34,10 @@
 typedef struct mca_btl_vader_fbox_t {
     union {
         struct {
-            uint8_t size;
+            uint16_t size;
-            uint8_t tag;
+            uint16_t tag;
         } hdr_data;
-        uint16_t ival;
+        uint32_t ival;
     } hdr;
 
     uint8_t data[MCA_BTL_VADER_FBOX_MAX_SIZE];