bsdacct: switch from global bsd_acct_struct instance to per-pidns one

Allocate the structure on the first call to sys_acct().  After this each
namespace, that ordered the accounting, will live with this structure till
its own death.

Two notes
- routines, that close the accounting on fs umount time use
  the init_pid_ns's acct by now;
- accounting routine accounts to dying task's namespace
  (also by now).

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Cc: Balbir Singh <balbir@in.ibm.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/pid_namespace.c

@@ -12,6 +12,7 @@
 #include <linux/pid_namespace.h>
 #include <linux/syscalls.h>
 #include <linux/err.h>
+#include <linux/acct.h>
 
 #define BITS_PER_PAGE		(PAGE_SIZE*8)
 
@@ -181,6 +182,7 @@ void zap_pid_ns_processes(struct pid_namespace *pid_ns)
 
 	/* Child reaper for the pid namespace is going away */
 	pid_ns->child_reaper = NULL;
+	acct_exit_ns(pid_ns);
 	return;
 }