md-cache: invalidate attributes on xattr update

xattr update will result in at least ctime change. So invalidate
attributes in xattr callback.

Change-Id: Ie6e8f2fd9a11c56c27e78bd58c2ff1e1d6edce6e
BUG: 953694
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/5641
Reviewed-by: Amar Tumballi <amarts@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>