In preparation to support multiple (aka chunked, aka splitted)
page I/O, this commit adds 'data_buff_pos' and 'oob_buff_pos' fields
to keep track of where the next read (or write) should be done.

This will allow multiple calls to handle_data_pio() to continue
the read (or write) operation.
Signed-off-by: NEzequiel Garcia <ezequiel.garcia@free-electrons.com>
Tested-by: NDaniel Mack <zonque@gmail.com>
Signed-off-by: NBrian Norris <computersforpeace@gmail.com>