uml: fix aio compilation bug

Restructure do_aio thanks to commments from Ulrich and Al.

Uli started this by seeing that UML's initialization of a struct iocb
initialized fields that it shouldn't.

Al followed up by adding the following cleanups:
	eliminating a variable by just using an anonymous structure in
its place.
	hoisting a duplicated line out of the switch.
	simplifying the error checking at the end.

I added a severity to the printk.

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Ulrich Drepper <drepper@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>