mm, meminit: replace rwsem with completion

Commit 0e1cc95b

 ("mm: meminit: finish initialisation of struct pages
before basic setup") introduced a rwsem to signal completion of the
initialization workers.

Lockdep complains about possible recursive locking:
  =============================================
  [ INFO: possible recursive locking detected ]
  4.1.0-12802-g1dc51b8 #3 Not tainted
  ---------------------------------------------
  swapper/0/1 is trying to acquire lock:
  (pgdat_init_rwsem){++++.+},
    at: [<ffffffff8424c7fb>] page_alloc_init_late+0xc7/0xe6

  but task is already holding lock:
  (pgdat_init_rwsem){++++.+},
    at: [<ffffffff8424c772>] page_alloc_init_late+0x3e/0xe6

Replace the rwsem by a completion together with an atomic
"outstanding work counter".

[peterz@infradead.org: Barrier removal on the grounds of being pointless]
[mgorman@suse.de: Applied review feedback]
Signed-off-by: Nicolai Stange <nicstange@gmail.com>
Signed-off-by: Mel Gorman <mgorman@suse.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Alex Ng <alexng@microsoft.com>
Cc: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>