fs: PM: Fix reverse check in filesystems_freeze_callback()

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.

Fixes: a3f8f8662771 ("power: always freeze efivarfs")
Cc: 6.18+ <stable@vger.kernel.org> # 6.18+
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://patch.msgid.link/12788397.O9o76ZdvQC@rafael.j.wysocki
Signed-off-by: Christian Brauner <brauner@kernel.org>