aio: allocate kiocbs in batches

In testing aio on a fast storage device, I found that the context lock
takes up a fair amount of cpu time in the I/O submission path.  The reason
is that we take it for every I/O submitted (see __aio_get_req).  Since we
know how many I/Os are passed to io_submit, we can preallocate the kiocbs
in batches, reducing the number of times we take and release the lock.

In my testing, I was able to reduce the amount of time spent in
_raw_spin_lock_irq by .56% (average of 3 runs).  The command I used to
test this was:

   aio-stress -O -o 2 -o 3 -r 8 -d 128 -b 32 -i 32 -s 16384 <dev>

I also tested the patch with various numbers of events passed to
io_submit, and I ran the xfstests aio group of tests to ensure I didn't
break anything.

Signed-off-by: Jeff Moyer <jmoyer@redhat.com>
Cc: Daniel Ehrenberg <dehrenberg@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/aio.h

@@ -117,6 +117,7 @@ struct kiocb {
 
 	struct list_head	ki_list;	/* the aio core uses this
 						 * for cancellation */
+	struct list_head	ki_batch;	/* batch allocation */
 
 	/*
 	 * If the aio_resfd field of the userspace iocb is not zero,