ADI: add MPID_Allocate_vci

This ADI allows MPIR layer to request for explicit vcis. The return must
be greater than 0 or inidicate failure. VCI 0 is the default global vci,
that is always used for implicit vci hashing..

src/mpid/ch3/include/mpidpre.h

@@ -532,6 +532,8 @@ int MPID_Init(int required, int *provided);
 
 int MPID_InitCompleted( void );
 
+int MPID_Allocate_vci(void);
+
 int MPID_Finalize(void);
 
 int MPID_Abort( MPIR_Comm *comm, int mpi_errno, int exit_code, const char *error_msg );

src/mpid/ch3/src/mpid_init.c

@@ -265,6 +265,12 @@ int MPID_InitCompleted( void )
     /* --END ERROR HANDLING-- */
 }
 
+int MPID_Allocate_vci(void)
+{
+    /* not supported, return 0 as failure */
+    return 0;
+}
+
 /*
  * Initialize the process group structure by using PMI calls.
  * This routine initializes PMI and uses PMI calls to setup the 

src/mpid/ch4/include/mpidch4.h

@@ -14,6 +14,7 @@
 
 int MPID_Init(int, int *);
 int MPID_InitCompleted(void);
+int MPID_Allocate_vci(void);
 MPL_STATIC_INLINE_PREFIX int MPID_Cancel_recv(MPIR_Request *) MPL_STATIC_INLINE_SUFFIX;
 MPL_STATIC_INLINE_PREFIX int MPID_Cancel_send(MPIR_Request *) MPL_STATIC_INLINE_SUFFIX;
 int MPID_Comm_disconnect(MPIR_Comm *);

src/mpid/ch4/src/ch4_init.c

@@ -518,6 +518,17 @@ int MPID_InitCompleted(void)
     goto fn_exit;
 }
 
+int MPID_Allocate_vci(void)
+{
+    if (MPIDI_global.n_vcis + MPIDI_global.n_reserved_vcis >= MPIDI_global.n_total_vcis) {
+        return 0;
+    } else {
+        MPIDI_global.n_reserved_vcis++;
+        /* we allocate from the back */
+        return (MPIDI_global.n_total_vcis - MPIDI_global.n_reserved_vcis);
+    }
+}
+
 int MPID_Finalize(void)
 {
     int mpi_errno;