LU-3546 mdt: define mdt_obd_name()

This patch defines the missing function mdt_obd_name()
in mdt_internal.h.

Signed-off-by: Bob Glossman <bob.glossman@intel.com>
Change-Id: I64d1b4df9f54e9ced7018f2c58067787a5dcf0a7
Signed-off-by: Jian Yu <jian.yu@intel.com>
Reviewed-on: http://review.whamcloud.com/6914
Tested-by: Hudson
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>