mm/vmalloc: take vmap_purge_lock in shrinker

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

mm/vmalloc: take vmap_purge_lock in shrinker

decay_va_pool_node() can be invoked concurrently from two paths:

__purge_vmap_area_lazy() when pools are being purged, and the shrinker via

vmap_node_shrink_scan().

However, decay_va_pool_node() is not safe to run concurrently, and the

shrinker path currently lacks serialization, leading to races and possible

leaks.

Protect decay_va_pool_node() by taking vmap_purge_lock in the shrinker

path to ensure serialization with purge users.