exfat: fix ctime is not updated

Commit 4c72a36edd54 removed attr_copy() from exfat_set_attr().
It causes xfstests generic/221 to fail. In xfstests generic/221,
it tests ctime should be updated even if futimens() update atime
only. But in this case, ctime will not be updated if attr_copy()
is removed.

attr_copy() may also update other attributes, and removing it may
cause other bugs, so this commit restores to call attr_copy() in
exfat_set_attr().

Fixes: 4c72a36edd54 ("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>