exfat: reuse exfat_inode_info variable instead of calling EXFAT_I()

Also add a local "struct exfat_inode_info *ei" variable to
exfat_truncate() to simplify the code.

Signed-off-by: Christophe Vu-Brugier <christophe.vu-brugier@seagate.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>