Avoid spurious directory metedata split brain

When directory content is modified, [mc]time is updated. On
Linux, the filesystem does it, while at least on NetBSD, the
kernel file-system independant code does it. This means that
when entries are added while bricks are down, the kernel sends
a SETATTR [mc]time which will cause metadata split brain for
the directory. In this case, clear the split brain by finding
the source with the most recent modification date.

Backport of: Ic0177e0df753a4748624d0b906834ed54593adb9

BUG: 1138897
Change-Id: Ic2e697be4f0074e2bbd3f23d6ad40a2d2d126a2c
Signed-off-by: Emmanuel Dreyfus <manu@netbsd.org>
Reviewed-on: http://review.gluster.org/9319
Reviewed-by: Krutika Dhananjay <kdhananj@redhat.com>
Reviewed-by: Ravishankar N <ravishankar@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Raghavendra Bhat <raghavendra@redhat.com>