PM: sleep: Drop spurious WARN_ON() from pm_restore_gfp_mask()

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

PM: sleep: Drop spurious WARN_ON() from pm_restore_gfp_mask()

Commit 35e4a69b2003f ("PM: sleep: Allow pm_restrict_gfp_mask()
stacking") introduced refcount-based GFP mask management that warns
when pm_restore_gfp_mask() is called with saved_gfp_count == 0.

Some hibernation paths call pm_restore_gfp_mask() defensively where
the GFP mask may or may not be restricted depending on the execution
path. For example, the uswsusp interface invokes it in
SNAPSHOT_CREATE_IMAGE, SNAPSHOT_UNFREEZE, and snapshot_release().
Before the stacking change this was a silent no-op; it now triggers
a spurious WARNING.

Remove the WARN_ON() wrapper from the !saved_gfp_count check while
retaining the check itself, so that defensive calls remain harmless
without producing false warnings.

[ rjw: Subject tweak ]