aio: fix misleading comments

The FIXME comments are inaccurate.
The locking comment over lookup_ioctx() is wrong.

Signed-off-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Zach Brown <zach.brown@oracle.com>
Signed-off-by: Shen Feng <shen@cn.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>