[LU-1756] compat code clean up

• Type: Improvement
• Resolution: Fixed
• Priority: Minor
• Fix Version/s: Lustre 2.4.0
• Labels: patch
• Assignee: Keith Mannthey (Inactive)
• Reporter: Peng Tao

Description

Clean up compat so that:
1. unused macros/functions are removed
2. redundant definitions are removed
3. client/server common functions are moved to lustre_common.h which is not compat code
4. use latest kernel functions and define functions for older kernel as wrappers to latest kernel APIs.