Things Not To Do When Writing A Driver, part 1001st:
have writev() and write() on the same file doing completely
different things.  As in, "interpret very different sets of
commands".

	We _can_ handle that, but it's a bloody bad idea.
Don't do that in new drivers.  Ever.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>