Fix race/oops in tty layer after BKL pushdown

While testing our KVM code for s390 (starting and killall kvm in a loop)
I can reproduce the following oops:

  Unable to handle kernel pointer dereference at virtual kernel address 6b6b6b6b6b6b6000 Oops: 0038 [#1] SMP
  Modules linked in: dm_multipath sunrpc qeth_l3 qeth_l2 dm_mod qeth
  ccwgroup CPU: 1 Not tainted 2.6.27-rc1 #54
  Process kuli (pid: 4409, task: 00000000b6aa5940, ksp: 00000000b7343e10)
  Krnl PSW : 0704e00180000000 00000000002e0b8c
  (disassociate_ctty+0x1c0/0x288) R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:0 AS:3
  CC:2 PM:0 EA:3 Krnl GPRS: 0000000000000000 6b6b6b6b6b6b6b6b
  0000000000000001 00000000000003a6 00000000002e0a46 00000000004b4160
  0000000000000001 00000000bbd79758 00000000b7343e58 00000000b8854148
  00000000bd34dea0 00000000b7343c20 0000000000000001 00000000004b6d08
  00000000002e0a46 00000000b7343c20 Krnl Code: 00000000002e0b7e:
  eb9fb0a00004	lmg	%r9,%r15,160(%r11) 00000000002e0b84:
  07f4		bcr	15,%r4 00000000002e0b86:
  e31090080004	lg	%r1,8(%r9) >00000000002e0b8c:
  d501109cd000	clc	156(2,%r1),0(%r13) 00000000002e0b92:
  a784ff5d		brc	8,2e0a4c 00000000002e0b96:
  b9040029		lgr	%r2,%r9 00000000002e0b9a:
  c0e5fffff9c3	brasl	%r14,2dff20 00000000002e0ba0:
  a7f4ff56		brc	15,2e0a4c Call Trace:
  ([<00000000002e0a46>] disassociate_ctty+0x7a/0x288)
   [<0000000000141fe6>] do_exit+0x212/0x8d4
   [<0000000000142708>] do_group_exit+0x60/0xcc
   [<0000000000150660>] get_signal_to_deliver+0x270/0x3ac
   [<000000000010bfd6>] do_signal+0x8e/0x8dc
   [<0000000000113772>] sysc_sigpending+0xe/0x22
   [<000001ff0000b134>] 0x1ff0000b134
  INFO: lockdep is turned off.
  Last Breaking-Event-Address:
   [<00000000002e0a48>] disassociate_ctty+0x7c/0x288
  Kernel panic - not syncing: Fatal exception: panic_on_oops

It seems that tty was already free in disassocate_ctty when it tries
to dereference tty->driver.

After moving the lock_kernel before the mutex_unlock, I can no longer
reproduce the problem.

[ This is a temporary partial fix for the documented and long standing
  race in disassociate_tty.  This stops most problem cases for now.

  For the next release the -next tree has an initial implementation of
  kref counting for tty structures and this quickfix will be dropped.

                                                              - Alan ]

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by; Alan Cox <alan@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>