fs: PM: Fix reverse check in filesystems_freeze_callback()

In the Linux kernel, the following vulnerability has been resolved:

fs: PM: Fix reverse check in filesystems_freeze_callback()

The freeze_all_ptr check in filesystems_freeze_callback() introduced by commit a3f8f8662771 ("power: always freeze efivarfs") is reverse which quite confusingly causes all file systems to be frozen when filesystem_freeze_enabled is false.

On my systems it causes the WARN_ON_ONCE() in __set_task_frozen() to trigger, most likely due to an attempt to freeze a file system that is not ready for that.

Add a logical negation to the check in question to reverse it as appropriate.