pid: remove pidhash

pidhash is no longer required as all the information can be looked up
from idr tree.  nr_hashed represented the number of pids that had been
hashed.  Since, nr_hashed and PIDNS_HASH_ADDING are no longer relevant,
it has been renamed to pid_allocated and PIDNS_ADDING respectively.

[gs051095@gmail.com: v6]
  Link: http://lkml.kernel.org/r/1507760379-21662-3-git-send-email-gs051095@gmail.com
Link: http://lkml.kernel.org/r/1507583624-22146-3-git-send-email-gs051095@gmail.com
Signed-off-by: Gargi Sharma <gs051095@gmail.com>
Reviewed-by: Rik van Riel <riel@redhat.com>
Tested-by: Tony Luck <tony.luck@intel.com>	[ia64]
Cc: Julia Lawall <julia.lawall@lip6.fr>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Pavel Tatashin <pasha.tatashin@oracle.com>
Cc: Kirill Tkhai <ktkhai@virtuozzo.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/fork.c

@@ -1871,7 +1871,7 @@ static __latent_entropy struct task_struct *copy_process(
 		retval = -ERESTARTNOINTR;
 		goto bad_fork_cancel_cgroup;
 	}
-	if (unlikely(!(ns_of_pid(pid)->nr_hashed & PIDNS_HASH_ADDING))) {
+	if (unlikely(!(ns_of_pid(pid)->pid_allocated & PIDNS_ADDING))) {
 		retval = -ENOMEM;
 		goto bad_fork_cancel_cgroup;
 	}