exfat: move exfat_entry_set_cache from heap to stack

The size of struct exfat_entry_set_cache is only 56 bytes on
64-bit system, and allocating from stack is more efficient than
allocating from heap.

Signed-off-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
Reviewed-by: Andy Wu <Andy.Wu@sony.com>
Reviewed-by: Aoyama Wataru <wataru.aoyama@sony.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>