coll/han: fix compiler warnings

Fix an unused variable, and explictly cast uint64_t* to size_t*

Note that the latter won't be needed after embiggening collectives for
large count support.

Signed-off-by: Wenduo Wang <wenduwan@amazon.com>

Author: wenduwan

ompi/mca/coll/han/coll_han_gatherv.c

@@ -332,7 +332,7 @@ int mca_coll_han_gatherv_intra(const void *sbuf, int scount, struct ompi_datatyp
     }
 
     if (max_data_size > (size_t) INT_MAX) {
-        datatype_size = coll_han_utils_gcd(low_data_size, low_size);
+        datatype_size = coll_han_utils_gcd((const size_t *) low_data_size, low_size);
     }
 
     low_rcounts = malloc(low_size * sizeof(int));

ompi/mca/coll/han/coll_han_reduce.c

@@ -132,7 +132,6 @@ mca_coll_han_reduce_intra(const void *sbuf,
     int *vranks = han_module->cached_vranks;
     int low_rank = ompi_comm_rank(low_comm);
     int low_size = ompi_comm_size(low_comm);
-    int up_rank  = ompi_comm_rank(up_comm);
 
     int root_low_rank;
     int root_up_rank;

ompi/mca/coll/han/coll_han_scatterv.c

@@ -343,7 +343,7 @@ int mca_coll_han_scatterv_intra(const void *sbuf, const int *scounts, const int
     }
 
     if (max_data_size > (size_t) INT_MAX) {
-        datatype_size = coll_han_utils_gcd(low_data_size, low_size);
+        datatype_size = coll_han_utils_gcd((const size_t *) low_data_size, low_size);
     }
 
     low_scounts = malloc(low_size * sizeof(int));