Considering this case:

 1. A program opens a sysfs table file 65535 times, it can increase
    validation_count and first increment cause the table to be mapped:

     validation_count = 65535

 2. AML execution causes "Load" to be executed on the same
    table, this time it cannot increase validation_count, so
    validation_count remains:

      validation_count = 65535

 3. The program closes sysfs table file 65535 times, it can decrease
    validation_count and the last decrement cause the table to be
    unmapped:

     validation_count = 0

 4. AML code still accessing the loaded table, kernel crash can be
    observed.

To prevent that from happening, add a validation_count threashold.
When it is reached, the validation_count can no longer be
incremented/decremented to invalidate the table descriptor (means
preventing table unmappings)

Note that code added in acpi_tb_put_table() is actually a no-op but
changes the warning message into a "warn once" one. Lv Zheng.
Signed-off-by: Lv Zheng <lv.zheng@intel.com>
[ rjw: Changelog, comments ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>