tty/ldsem: Wake up readers after timed out down_write()

ldsem_down_read() will sleep if there is pending writer in the queue.
If the writer times out, readers in the queue should be woken up,
otherwise they may miss a chance to acquire the semaphore until the last
active reader will do ldsem_up_read().

There was a couple of reports where there was one active reader and
other readers soft locked up:
  Showing all locks held in the system:
  2 locks held by khungtaskd/17:
   #0:  (rcu_read_lock){......}, at: watchdog+0x124/0x6d1
   #1:  (tasklist_lock){.+.+..}, at: debug_show_all_locks+0x72/0x2d3
  2 locks held by askfirst/123:
   #0:  (&tty->ldisc_sem){.+.+.+}, at: ldsem_down_read+0x46/0x58
   #1:  (&ldata->atomic_read_lock){+.+...}, at: n_tty_read+0x115/0xbe4

Prevent readers wait for active readers to release ldisc semaphore.

Link: lkml.kernel.org/r/[email protected]
Link: lkml.kernel.org/r/20180907045041.GF1110@shao2-debian
Cc: Jiri Slaby <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: [email protected]
Reported-by: kernel test robot <[email protected]>
Signed-off-by: Dmitry Safonov <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

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