LU-2479 ldiskfs: do not check dir max size for regular files

ldiskfs_append() is used not only to extend directory but also to
maintain iam container. In later case ldiskfs_append() should not
check for max directory size. The iam container is distunguished as
regular file.

This is a backport of
Lustre-commit: 4b9e64aec7cfdf859f1f931e1ee44056db050bb9
Lustre-change: http://review.whamcloud.com/8137

Signed-off-by: Vladimir Saveliev <vladimir_saveliev@xyratex.com>
Signed-off-by: Emoly Liu <emoly.liu@intel.com>
Change-Id: Ibc3e96078e9af3eb854c6666f087b1049b68f409
Reviewed-on: http://review.whamcloud.com/10043
Tested-by: Jenkins
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: James Nunez <james.a.nunez@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>