exfat: reduce the size of exfat_entry_set_cache

In normal, there are 19 directory entries at most for a file or
a directory.
  - A file directory entry
  - A stream extension directory entry
  - 1~17 file name directory entry

So the directory entries are in 3 sectors at most, it is enough
for struct exfat_entry_set_cache to pre-allocate 3 bh.

This commit changes the size of struct exfat_entry_set_cache as:

                   Before   After
32-bit system      88       32    bytes
64-bit system      168      48    bytes

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>
Reviewed-by: Sungjong Seo <sj1557.seo@samsung.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>