r8169: bh locking redux and task scheduling.

r8169: bh locking redux and task scheduling.

- atomic bit operations are globally visible
- pending status is always cleared before execution
- scheduled works are either idempotent or only required to happen once
  after a series of originating events, say link events for instance

(cherry picked from commit 98ddf986fca17840e46e070354b7e2cd2169da15)
Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
Suggested-by: Michał Mirosław <mirqus@gmail.com>
Cc: Hayes Wang <hayeswang@realtek.com>
Signed-off-by: Joe Jin <joe.jin@oracle.com>