ITER_PIPE: allocate buffers as we go in copy-to-pipe primitives

New helper: append_pipe().  Extends the last buffer if possible,
allocates a new one otherwise.  Returns page and offset in it
on success, NULL on failure.  iov_iter is advanced past the
data we've got.

Use that instead of push_pipe() in copy-to-pipe primitives;
they get simpler that way.  Handling of short copy (in "mc" one)
is done simply by iov_iter_revert() - iov_iter is in consistent
state after that one, so we can use that.

[Fix for braino caught by Liu Xinpeng <[email protected]> folded in]
[another braino fix, this time in copy_pipe_to_iter() and pipe_zero();
caught by testcase from Hugh Dickins]

Signed-off-by: Al Viro <[email protected]>

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