LU-1319 mdt: increment MDT getattr stats

LU-1319 mdt: increment MDT getattr stats

Move increment of MDT getattr stat from mdt_getattr() to
mdt_getattr_internal() so we don't miss other call paths
that may service getattr requests.

Signed-off-by: Ned Bass <bass6@llnl.gov>
Change-Id: I45ae7a73eaf81fbe56775725d07ec275b4261f1f
Reviewed-on: http://review.whamcloud.com/2526
Reviewed-by: Yang Sheng <ys@whamcloud.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Fan Yong <yong.fan@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>