mm/vmalloc: free unused pages on vrealloc() shrink

When vrealloc() shrinks an allocation and the new size crosses a page
boundary, unmap and free the tail pages that are no longer needed.  This
reclaims physical memory that was previously wasted for the lifetime of
the allocation.

The heuristic is simple: always free when at least one full page becomes
unused.  Huge page allocations (page_order > 0) are skipped, as partial
freeing would require splitting.  Allocations with VM_FLUSH_RESET_PERMS
are also skipped, as their direct-map permissions must be reset before
pages are returned to the page allocator, which is handled by
vm_reset_perms() during vfree().

Additionally, allocations with VM_USERMAP are skipped because
remap_vmalloc_range_partial() validates mapping requests against the
unchanged vm->size; freeing tail pages would cause vmalloc_to_page() to
return NULL for the unmapped range.

To protect concurrent readers, the shrink path uses Node lock to
synchronize before freeing the pages.

Finally, we notify kmemleak of the reduced allocation size using
kmemleak_free_part() to prevent the kmemleak scanner from faulting on the
newly unmapped virtual addresses.

The virtual address reservation (vm->size / vmap_area) is intentionally
kept unchanged, preserving the address for potential future grow-in-place
support.

Link: https://lore.kernel.org/20260519-vmalloc-shrink-v14-4-70b96ee3e9c9@zohomail.in
Signed-off-by: Shivam Kalra <shivamkalra98@zohomail.in>
Suggested-by: Danilo Krummrich <dakr@kernel.org>
Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Cc: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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