fs: Asynchronous I/O latency to a solid-state disk greatly increased

between the 2.6.32 and 3.0 kernels. By removing the plug from
do_io_submit(), we observed a 34% improvement in the I/O latency.

Unfortunately, at this level, we don't know if the request is to
a rotating disk or not.

Change-Id: Ifca5f0e5c58bcb9e823f3b71927fb326866587e6
Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>