fix: set type of mask_val

this change ensures the type of `mask_val` is correctly set; otherwise,
we risk transfering a 4-byte value to arrays of 4-byte elements with a
size of 2 (i.e., `i4tmp(2)` and `r4tmp(2)`)

mpp/include/mpp_chksum_int.fh

@@ -51,7 +51,7 @@ function MPP_CHKSUM_INT_RMASK_( var, pelist, mask_val )
   integer(KIND=i8_kind) :: MPP_CHKSUM_INT_RMASK_
   MPP_TYPE_, intent(in) :: var MPP_RANK_
   integer, optional :: pelist(:)
-  real, intent(in) :: mask_val
+  real(KIND=r8_kind), intent(in) :: mask_val
   integer(KIND(var))::imask_val
   integer(KIND=i4_kind)::i4tmp(2)=0
   real(KIND=r4_kind)::r4tmp(2)=0