exfat: fix memory leak in exfat_load_bitmap()

If the first directory entry in the root directory is not a bitmap
directory entry, 'bh' will not be released and reassigned, which
will cause a memory leak.

Fixes: 1e49a94c ("exfat: add bitmap operations")
Cc: stable@vger.kernel.org
Signed-off-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
Reviewed-by: Aoyama Wataru <wataru.aoyama@sony.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>