iov_iter work, part 1 - isolated cleanups and optimizations.

	One of the goals is to reduce the overhead of using ->read_iter()
and ->write_iter() instead of ->read()/->write(); new_sync_{read,write}()
has a surprising amount of overhead, in particular inside iocb_flags().
That's why the beginning of the series is in this pile; it's not directly
iov_iter-related, but it's a part of the same work...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
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