fatfs: fix NULL deref in fat32 code

setdstart() was checking ISFAT32(dpbp) before dpbp was initialized.
This produced various crashes and inconsistencies.
Just remove the check and store the full cluster count.
On fat16 it shouldn't exceed 16 bits anyway.

kernel/fatfs.c

@@ -69,8 +69,7 @@ CLUSTER getdstart(struct dpb FAR *dpbp, struct dirent *dentry)
 void setdstart(struct dpb FAR *dpbp, struct dirent *dentry, CLUSTER value)
 {
   dentry->dir_start = (UWORD)value;
-  if (ISFAT32(dpbp))
-    dentry->dir_start_high = (UWORD)(value >> 16);
+  dentry->dir_start_high = (UWORD)(value >> 16);
 }
 #endif