mkfatfs: use DMA memory for mkfatfs when needed

this makes mkfatfs use fat_dma_alloc() when CONFIG_FAT_DMAMEMORY is
set. This is needed to ensure mkfatfs operates with boards that use
DMA for microSD

From Andrew Tridgell via the PX4/NuttX repository

fs/fat/fs_mkfatfs.c

@@ -298,7 +298,12 @@ int mkfatfs(FAR const char *pathname, FAR struct fat_format_s *fmt)
 
   /* Allocate a buffer that will be working sector memory */
 
+#ifdef CONFIG_FAT_DMAMEMORY
+  var.fv_sect = (uint8_t*)fat_dma_alloc(var.fv_sectorsize);
++#else
   var.fv_sect = (uint8_t*)kmm_malloc(var.fv_sectorsize);
+#endif
+
   if (!var.fv_sect)
     {
       fdbg("ERROR: Failed to allocate working buffers\n");
@@ -319,7 +324,11 @@ errout:
 
   if (var.fv_sect)
     {
+#ifdef CONFIG_FAT_DMAMEMORY
+      fat_dma_free(var.fv_sect, var.fv_sectorsize);
+#else
       kmm_free(var.fv_sect);
+#endif
     }
 
   /* Return any reported errors */