ACPI / IPMI: Fix race caused by the unprotected ACPI IPMI user

This patch uses reference counting to fix the race caused by the
unprotected ACPI IPMI user.

There are two rules for using the ipmi_si APIs:
 1. In ipmi_si, ipmi_destroy_user() can ensure that no ipmi_recv_msg will
    be passed to ipmi_msg_handler(), but ipmi_request_settime() can not
    use an invalid ipmi_user_t.  This means the ipmi_si users must ensure
    that there won't be any local references on ipmi_user_t before invoking
    ipmi_destroy_user().
 2. In ipmi_si, the smi_gone()/new_smi() callbacks are protected by
    smi_watchers_mutex, so their execution is serialized.  But as a
    new smi can re-use a freed intf_num, it requires that the callback
    implementation must not use intf_num as an identification mean or it
    must ensure all references to the previous smi are all dropped before
    exiting smi_gone() callback.

As the acpi_ipmi_device->user_interface check in acpi_ipmi_space_handler()
can happen before setting user_interface to NULL and codes after the check
in acpi_ipmi_space_handler() can happen after user_interface becomes NULL,
the on-going acpi_ipmi_space_handler() still can pass an invalid
acpi_ipmi_device->user_interface to ipmi_request_settime().  Such race
conditions are not allowed by the IPMI layer's API design as a crash will
happen in ipmi_request_settime() if something like that happens.

This patch follows the ipmi_devintf.c design:
 1. Invoke ipmi_destroy_user() after the reference count of
    acpi_ipmi_device drops to 0.  References of acpi_ipmi_device dropping
    to 0 also means tx_msg related to this acpi_ipmi_device are all freed.
    This matches the IPMI layer's API calling rule on ipmi_destroy_user()
    and ipmi_request_settime().
 2. ipmi_flush_tx_msg() is performed so that no on-going tx_msg can still be
    running in acpi_ipmi_space_handler().  And it is invoked after invoking
    __ipmi_dev_kill() where acpi_ipmi_device is deleted from the list with a
    "dead" flag set, and the "dead" flag check is also introduced to the
    point where a tx_msg is going to be added to the tx_msg_list so that no
    new tx_msg can be created after returning from the __ipmi_dev_kill().
 3. The waiting codes in ipmi_flush_tx_msg() is deleted because it is not
    required since this patch ensures no acpi_ipmi reference is still held
    for ipmi_user_t before calling ipmi_destroy_user() and
    ipmi_destroy_user() can ensure no more ipmi_msg_handler() can happen
    after returning from ipmi_destroy_user().
 4. The flushing of tx_msg is also moved out of ipmi_lock in this patch.

The forthcoming IPMI operation region handler installation changes also
requires acpi_ipmi_device be handled in this style.

The header comment of the file is also updated due to this design change.

Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Reviewed-by: Huang Ying <ying.huang@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>