lustre/mdc: comments on LOOKUP and PERM lock

Add more comments for MDS_INODELOCK_PERM and
MDS_INODELOCK_LOOKUP

Signed-off-by: wang di <di.wang@intel.com>
Reviewed-on: http://review.whamcloud.com/7937
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-3240


Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>