new helper: copy_page_from_iter()

parallel to copy_page_to_iter().  pipe_write() switched to it (and became
->write_iter()).

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

diff --cc fs/pipe.c
Simple merge

diff --cc include/linux/uio.h
Simple merge

diff --cc mm/iov_iter.c
Simple merge