exfat: fix setting uninitialized time to ctime/atime

An uninitialized time is set to ctime/atime in __exfat_write_inode().
It causes xfstests generic/003 and generic/192 to fail.

And since there will be a time gap between setting ctime/atime to
the inode and writing back the inode, so ctime/atime should not be
set again when writing back the inode.

Fixes: 4c72a36e ("exfat: convert to new timestamp accessors")
Signed-off-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
Reviewed-by: Andy Wu <Andy.Wu@sony.com>
Reviewed-by: Aoyama Wataru <wataru.aoyama@sony.com>
Reviewed-by: Sungjong Seo <sj1557.seo@samsung.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>