tty: fix pid memleak in disassociate_ctty()

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I7PBYJ



--------------------------------

There is memleak in alloc_pid:
------------------------------
unreferenced object 0xffff88810c181940 (size 224):
  comm "sshd", pid 8191, jiffies 4294946950 (age 524.570s)
  hex dump (first 32 bytes):
    01 00 00 00 00 00 00 00 00 00 00 00 ad 4e ad de  .............N..
    ff ff ff ff 6b 6b 6b 6b ff ff ff ff ff ff ff ff  ....kkkk........
  backtrace:
    [<ffffffff814774e6>] kmem_cache_alloc+0x5c6/0x9b0
    [<ffffffff81177342>] alloc_pid+0x72/0x570
    [<ffffffff81140ac4>] copy_process+0x1374/0x2470
    [<ffffffff81141d77>] kernel_clone+0xb7/0x900
    [<ffffffff81142645>] __se_sys_clone+0x85/0xb0
    [<ffffffff8114269b>] __x64_sys_clone+0x2b/0x30
    [<ffffffff83965a72>] do_syscall_64+0x32/0x80
    [<ffffffff83a00085>] entry_SYSCALL_64_after_hwframe+0x61/0xc6

The pid memleak is triggered by the following race:
task[sshd]                      task[bash]
-----------------------		-----------------------
				do_exit();
				disassociate_ctty();
				spin_lock_irq(¤t->sighand->siglock);
				put_pid(current->signal->tty_old_pgrp);
				current->signal->tty_old_pgrp = NULL;
				tty = tty_kref_get(current->signal->tty);
				//tty is not NULL
				spin_unlock_irq(¤t->sighand->siglock);
tty_vhangup();
tty_lock(tty);
...
tty_signal_session_leader();
spin_lock_irq(&p->sighand->siglock);
...
p->signal->tty_old_pgrp = get_pid(tty->pgrp); // tty_old_pgrp reassign
spin_unlock_irq(&p->sighand->siglock);
...
tty_unlock(tty);
				if (tty) {
				    tty_lock(tty);
				    ...
				    put_pid(tty->pgrp);
				    tty->pgrp = NULL;// It's too late
				    ...
				    tty_unlock(tty);
				}

in task[bash], tty_old_pgrp is released by disassociate_ctty(), then it's
reassigned by tty_signal_session_leader() in task[sshd], cause memleak.

fix the memleak by add put_pid() in disassociate_ctty() after tty_old_pgrp
is reassigned.

Fixes: c8bcd9c5 ("tty: Fix ->session locking")
Signed-off-by: Yi Yang <yiyang13@huawei.com>