switch default_file_splice_read() to use of pipe-backed iov_iter

we only use iov_iter_get_pages_alloc() and iov_iter_advance() -
pages are filled by kernel_readv() via a kvec array (as we used
to do all along), so iov_iter here is used only as a way of
arranging for those pages to be in pipe.

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