When an AIO write gets an error after writing some data (eg.  ENOSPC), it
should return the amount written already, not the error.  Just like write()
is supposed to.

This was found by the libaio test suite.
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>
Acked-By: NZach Brown <zach.brown@oracle.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>