extmod/modos: Only sync FAT filesystems using disk_ioctl.

Eventually this needs to be made a generic call to the underlying VFS.  But
for now this prevents `disk_ioctl()` crashing on non-FAT filesystems.

Signed-off-by: Damien George <damien@micropython.org>

extmod/modos.c

@@ -76,9 +76,10 @@
 static mp_obj_t mp_os_sync(void) {
     #if MICROPY_VFS_FAT
     for (mp_vfs_mount_t *vfs = MP_STATE_VM(vfs_mount_table); vfs != NULL; vfs = vfs->next) {
-        // this assumes that vfs->obj is fs_user_mount_t with block device functions
+        if (mp_obj_is_type(vfs->obj, &mp_fat_vfs_type)) {
             disk_ioctl(MP_OBJ_TO_PTR(vfs->obj), CTRL_SYNC, NULL);
         }
+    }
     #endif
     return mp_const_none;
 }