io_uring: mark cold functions

Attribute cold functions so compilers can optimise them for size. It
shrinks the binary by 2.5-3%

   text    data     bss     dec     hex filename
  90670   14002       8  104680   198e8 ./fs/io_uring.o
  88053   14002       8  102063   18eaf ./fs/io_uring.o
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/b53d385f91dca45170b67d7f11c7abd787e821f6.1633373302.git.asml.silence@gmail.comSigned-off-by: Jens Axboe <axboe@kernel.dk>