exfat: Fix potential use after free in exfat_load_upcase_table()

This code calls brelse(bh) and then dereferences "bh" on the next line
resulting in a possible use after free.  The brelse() should just be
moved down a line.

Fixes: b676fdbcf4c8 ("exfat: standardize checksum calculation")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>