When invoking kexec() on a Linux guest running on a Hyper-V host, the
kernel panics.

    RIP: 0010:cpuhp_issue_call+0x137/0x140
    Call Trace:
    __cpuhp_remove_state_cpuslocked+0x99/0x100
    __cpuhp_remove_state+0x1c/0x30
    hv_kexec_handler+0x23/0x30 [hv_vmbus]
    hv_machine_shutdown+0x1e/0x30
    machine_shutdown+0x10/0x20
    kernel_kexec+0x6d/0x96
    __do_sys_reboot+0x1ef/0x230
    __x64_sys_reboot+0x1d/0x20
    do_syscall_64+0x6b/0x3d8
    entry_SYSCALL_64_after_hwframe+0x44/0xa9

This was due to hv_synic_cleanup() callback returning -EBUSY to
cpuhp_issue_call() when tearing down the VMBUS_CONNECT_CPU, even
if the vmbus_connection.conn_state = DISCONNECTED. hv_synic_cleanup()
should succeed in the case where vmbus_connection.conn_state
is DISCONNECTED.

Fix is to add an extra condition to test for
vmbus_connection.conn_state == CONNECTED on the VMBUS_CONNECT_CPU and
only return early if true. This way the kexec() path can still shut
everything down while preserving the initial behavior of preventing
CPU offlining on the VMBUS_CONNECT_CPU while the VM is running.

Fixes: 8a857c55 ("Drivers: hv: vmbus: Always handle the VMBus messages on CPU0")
Signed-off-by: Chris Co <chrco@microsoft.com>
Reviewed-by: Andrea Parri (Microsoft) <parri.andrea@gmail.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20201110190118.15596-1-chrco@linux.microsoft.comSigned-off-by: Wei Liu <wei.liu@kernel.org>