vulkan/malloc: fix data race on vk_malloc.age

TSAN reports a data race between vk_malloc_garbage_collect and
vk_malloc_slice, the former increments vk_malloc.age while holding the
vk_malloc.lock while the latter reads it after dropping the lock.

Fix this by setting slab->age inside pool_get_page when returning
already allocated slab. slab_alloc already sets it for new slabs.

TSAN trace:
WARNING: ThreadSanitizer: data race (pid=847921)
Write of size 8 at 0x725400013878 by thread T15 (mutexes: write M0, write M1):
\#0 vk_malloc_garbage_collect /mnt/f/workspace/libplacebo/build/../src/vulkan/malloc.c:608:12
\#1 vk_gpu_flush /mnt/f/workspace/libplacebo/build/../src/vulkan/gpu.c:756:5
\#2 pl_gpu_flush /mnt/f/workspace/libplacebo/build/../src/gpu.c:1267:9

Previous read of size 8 at 0x725400013878 by main thread (mutexes: write M2, write M3):
\#0 vk_malloc_slice /mnt/f/workspace/libplacebo/build/../src/vulkan/malloc.c:1063:25
\#1 vk_buf_create /mnt/f/workspace/libplacebo/build/../src/vulkan/gpu_buf.c:221:10
\#2 pl_buf_create /mnt/f/workspace/libplacebo/build/../src/gpu.c:609:18

Author: llyyr

src/vulkan/malloc.c

@@ -742,6 +742,7 @@ static struct vk_slab *pool_get_page(struct vk_malloc *ma, struct vk_pool *pool,
 
         slab->spacemap ^= 0x1LLU << page_idx;
         *offset = page_idx * slab->pagesize;
+        slab->age = ma->age;
         return slab;
     }
 
@@ -1060,7 +1061,6 @@ bool vk_malloc_slice(struct vk_malloc *ma, struct vk_memslice *out,
         // consumers are always aligned properly.
         size = PL_ALIGN(size, align);
         slab->used += size;
-        slab->age = ma->age;
         if (params->debug_tag)
             slab->debug_tag = params->debug_tag;
         pl_mutex_unlock(&slab->lock);