tty: protect tty_write from odd low-level tty disciplines

Al root-caused a new warning from syzbot to the ttyprintk tty driver
returning a write count larger than the data the tty layer actually gave
it.  Which confused the tty write code mightily, and with the new
iov_iter based code, caused a WARNING in iov_iter_revert().

syzbot correctly bisected the source of the new warning to commit
9bb48c82

 ("tty: implement write_iter"), but the oddity goes back
much further, it just didn't get caught by anything before.

Reported-by:  <syzbot+3d2c27c2b7dc2a94814d@syzkaller.appspotmail.com>
Fixes: 9bb48c82

 ("tty: implement write_iter")
Debugged-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>