exfat: use i_blocksize() to get blocksize

We alreday has the interface i_blocksize() to get blocksize,
so use it.
Signed-off-by: Xianting Tian <tian.xianting@h3c.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>

fs/exfat/file.c

@@ -229,7 +229,7 @@ void exfat_truncate(struct inode *inode, loff_t size)
 {
 	struct super_block *sb = inode->i_sb;
 	struct exfat_sb_info *sbi = EXFAT_SB(sb);
-	unsigned int blocksize = 1 << inode->i_blkbits;
+	unsigned int blocksize = i_blocksize(inode);
 	loff_t aligned_size;
 	int err;