xenial: soft lockup in check_unsafe_exec()

Endless cycle was detected few times during ubuntu 16.04 testing insied openvz VMs
dmesg output and crash dump ivestigation shows that foolowing ubuntu 16.04.patch is guilty
commit d6572202d986a84feda4cf7e6ce0bebaa18cb8fe
Author: Colin Ian King <email address hidden>
Date: Fri May 12 15:51:56 2017 +0100
    UBUNTU:SAUCE: exec: ensure file system accounting in check_unsafe_exec is correct

    BugLink: http://bugs.launchpad.net/bugs/1672819

fs/exec.c:: check_unsafe_exec()
...
recheck:
        fs_recheck = false;
        t = p;
        n_fs = 1;
        spin_lock(&p->fs->lock);
        rcu_read_lock();
        while_each_thread(p, t) {
                if (t->fs == p->fs)
                        n_fs++;
                if (t->flags & (PF_EXITING | PF_FORKNOEXEC))
                        fs_recheck = true;
        }
        rcu_read_unlock();

        if (p->fs->users > n_fs) {
                if (fs_recheck) {
                        spin_unlock(&p->fs->lock);
                        goto recheck; <<<<<< cycles forever
                }
                bprm->unsafe |= LSM_UNSAFE_SHARE;
        } else
                p->fs->in_exec = 1;
        spin_unlock(&p->fs->lock);

dmesg example is attached,
let me know if you want to look at crash dump image.
Bug is xenial-specific (4.4 kernel), other kernel branches seems are not affected.