It's expensive to set buffer flags that are already set, because that
causes a costly cache line transition.

A common case is setting the "verified" flag during ext4 writes.
This patch checks for the flag being set first.

With the AIM7/creat-clo benchmark testing on a 48G ramdisk based-on ext4
file system, we see 3.3%(15431->15936) improvement of aim7.jobs-per-min on
a 2-sockets broadwell platform.

What the benchmark does is: it forks 3000 processes, and each  process do
the following:
a) open a new file
b) close the file
c) delete the file
until loop=100*1000 times.

The original patch is contributed by Andi Kleen.
Signed-off-by: NAndi Kleen <ak@linux.intel.com>
Tested-by: NKemi Wang <kemi.wang@intel.com>
Signed-off-by: NKemi Wang <kemi.wang@intel.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>