"sshd" tasks blocked on tty_flush_to_ldisc() waiting for "event/1" thread.

Issue

• Server hung with multiple "sshd" tasks blocked in uninterruptable state (UN/D).
• Multiple "sshd" tasks blocked with following backtrace;

PID: 27292  TASK: ffff88037ccb0ae0  CPU: 2   COMMAND: "sshd"
 #0 [ffff88037cc216a8] schedule at ffffffff8150e172
 #1 [ffff88037cc21770] schedule_timeout at ffffffff8150f035
 #2 [ffff88037cc21820] wait_for_common at ffffffff8150ecb3
 #3 [ffff88037cc218b0] wait_for_completion at ffffffff8150edcd
 #4 [ffff88037cc218c0] flush_work at ffffffff810917f7
 #5 [ffff88037cc21930] flush_delayed_work at ffffffff81091a14
 #6 [ffff88037cc21950] tty_flush_to_ldisc at ffffffff8132d955
 #7 [ffff88037cc21960] n_tty_poll at ffffffff813285a7
 #8 [ffff88037cc21990] tty_poll at ffffffff8132416a
 #9 [ffff88037cc219d0] do_select at ffffffff81198392
#10 [ffff88037cc21d70] core_sys_select at ffffffff8119884a
#11 [ffff88037cc21f10] sys_select at ffffffff81198bd7
#12 [ffff88037cc21f80] system_call_fastpath at ffffffff8100b072
    RIP: 00007f65710344d3  RSP: 00007fff49d59108  RFLAGS: 00010202
    RAX: 0000000000000017  RBX: ffffffff8100b072  RCX: 000000000000006e
    RDX: 00007f6575286df0  RSI: 00007f6575286e10  RDI: 000000000000000a
    RBP: 0000000000000000   R8: 0000000000000000   R9: 0101010101010101
    R10: 0000000000000000  R11: 0000000000000246  R12: 00007fff49d591a0
    R13: 00007fff49d591a4  R14: 00007f6573edc67c  R15: 00007fff49d59190
    ORIG_RAX: 0000000000000017  CS: 0033  SS: 002b