task IO accounting: move all IO statistics in struct task_io_accounting

Simplify the code of include/linux/task_io_accounting.h.

It is also more reasonable to have all the task i/o-related statistics in a
single struct (task_io_accounting).

Signed-off-by: Andrea Righi <righi.andrea@gmail.com>
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>