[PATCH] aio: optimize io_submit_one()

This patch optimizes io_submit_one to call aio_run_iocb() directly if
ctx->run_list is empty.  When the list is empty, the operation of adding to
the list, then call to __aio_run_iocbs() is unnecessary because these
operations are done in one atomic step.  ctx->run_list always has only one
element in this case.  This optimization speeds up industry standard db
transaction processing benchmark by 0.2%.

Signed-off-by: Ken Chen <kenneth.w.chen@intel.com>
Cc: Benjamin LaHaise <bcrl@kvack.org>
Cc: Suparna Bhattacharya <suparna@in.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>