TTY: fix atime/mtime regression

In commit b0de59b5 ("TTY: do not update atime/mtime on read/write")
we removed timestamps from tty inodes to fix a security issue and waited
if something breaks.  Well, 'w', the utility to find out logged users
and their inactivity time broke.  It shows that users are inactive since
the time they logged in.

To revert to the old behaviour while still preventing attackers to
guess the password length, we update the timestamps in one-minute
intervals by this patch.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>