Release pageheap_lock while releasing memory to the OS from HugePageFiller.

[copybara-service]

Release pageheap_lock while releasing memory to the OS from HugePageFiller.

Drop pageheap_lock around unback syscalls in HugePageFiller::ReleasePages (via UnbackSubreleaseFunction) and HugePageFiller::HandleFullyFreedTracker while preserving tracker invariants against concurrent allocations and deallocations:

• Remove the PageTracker from regular_alloc_ / regular_alloc_partial_released_ before invoking ReleaseFree, temporarily pinning the subreleasing range in RangeTracker so concurrent TryGet calls skip it and concurrent Put calls cannot return the tracker as fully freed while unback is in flight.
• Re-evaluate tracker state upon reacquiring pageheap_lock after ReleaseFree: if empty, defer whole-hugepage unback via StoreFullyFreedTracker for the caller to drain via HandleFullyFreedTracker; otherwise reinsert into the appropriate tracker list.
• Stop multi-range ReleaseFree loops immediately if concurrent Put calls empty the tracker during an unlocked unback.
• Update n_was_released_ and RecordLifetime before dropping pageheap_lock in HandleFullyFreedTracker so filler statistics remain consistent during unlocked unback.