exfat: add missing brelse() calls on error paths

If the second exfat_get_dentry() call fails then we need to release
"old_bh" before returning.  There is a similar bug in exfat_move_file().

Fixes: 5f2aa075 ("exfat: add inode operations")
Reported-by: Markus Elfring <Markus.Elfring@web.de>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>