sparse fixes for io_uring

Try to make sparse a little more happy.  The poll code is still a major
trainwreck, but that will need more intensive care.

Follow-Ups:
- [PATCH 5/6] io_uring: consistently use the EPOLL* defines
  - From: Christoph Hellwig
- [PATCH 6/6] io_uring: use rcu_dereference in io_close
  - From: Christoph Hellwig
- [PATCH 2/6] io_uring: don't use ERR_PTR for user pointers
  - From: Christoph Hellwig
- [PATCH 1/6] io_uring: use a rwf_t for io_rw.flags
  - From: Christoph Hellwig
- [PATCH 4/6] io_uring: make apoll_events a __poll_t
  - From: Christoph Hellwig
- [PATCH 3/6] io_uring: drop a spurious inline on a forward declaration
  - From: Christoph Hellwig