tty: implement read_iter

Now that the ldisc read() function takes kernel pointers, it's fairly
straightforward to make the tty file operations use .read_iter() instead
of .read().

That automatically gives us vread() and friends, and also makes it
possible to do .splice_read() on ttys again.

Fixes: 36e2c742 ("fs: don't allow splice read/write without explicit ops")
Reported-by: Oliver Giles <ohw.giles@gmail.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>