io_uring: inline io_req_task_work_add()

__io_req_task_work_add() is huge but marked inline, that makes compilers
to generate lots of garbage. Inline the wrapper caller
io_req_task_work_add() instead.

before and after:
   text    data     bss     dec     hex filename
  47347   16248       8   63603    f873 io_uring/io_uring.o
   text    data     bss     dec     hex filename
  45303   16248       8   61559    f077 io_uring/io_uring.o

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/26dc8c28ca0160e3269ef3e55c5a8b917c4d4450.1668162751.git.asml.silence@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>