LU-4973 mdd: only check links limit for non-directories

LU-4973 mdd: only check links limit for non-directories

Since directory(for ldiskfs) might exceed maximum link count, so we
do not check maximum link count for dir in MDD layer.

Also add a few comments to mdd_may_xxx() to describe the input
parameters and return values.

Clean up a couple of error messages that are spewed when doing
invalid namespace operations.

Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: wang di <di.wang@intel.com>
Change-Id: I36c354f6f05f1c0fb8f632dfc23f8e2ef63ebbe5
Reviewed-on: http://review.whamcloud.com/10150
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>