iov_iter: saner checks for attempt to copy to/from iterator

iov_iter: saner checks for attempt to copy to/from iterator

instead of "don't do it to ITER_PIPE" check for ->data_source being
false on copying from iterator.  Check for !->data_source for
copying to iterator, while we are at it.

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