- pass gfp_flags to get_io_context(): not all callers are forced to use
  GFP_ATOMIC().

- fix locking in get_io_context(): bump the refcount whilein the exclusive
  region.

- don't go oops in get_io_context() if the kmalloc failed.

- in as_get_io_context(): fail the whole thing if we were unable to
  allocate the AS-specific part.

- as_remove_queued_request() cleanup