nsproxy: move free_nsproxy() out of do_exit() path

nsproxy: move free_nsproxy() out of do_exit() path

free_nsproxy() is too heavy to be on the exit path.  Let's free namespaces
asynchronously to not block exit_group() syscall.

Microbenchmark:

: #define _GNU_SOURCE
: #include <unistd.h>
: #include <sched.h>
: #include <stdlib.h>
: #include <sys/wait.h>
:
: int
: main(void)
: {
:       int i;
:       for (i = 0; i < 1024; i++) {
:               if (fork()) {
:                       wait(NULL);
:                       continue;
:               }
:               unshare(CLONE_NEWIPC);
:               exit(0);
:       }
:       return 0;
: }

Before the patch:

real    0m8.335s
user    0m0.000s
sys     0m0.265s

After:

real    0m0.569s
user    0m0.001s
sys     0m0.154s

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: "Dmitry V. Levin" <ldv@altlinux.org>
Cc: Pavel Emelyanov <xemul@openvz.org>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Doug Ledford <dledford@redhat.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>