slab: Add allow_spin check to eliminate kmemleak warnings

In slab_post_alloc_hook(), kmemleak check is skipped when
gfpflags_allow_spinning() returns false since commit af92793e
("slab: Introduce kmalloc_nolock() and kfree_nolock().")
Therefore, unconditionally calling kmemleak_not_leak() in
alloc_slab_obj_exts() would trigger the following warning:

kmemleak: Trying to color unknown object at 0xffff8881057f5000 as Grey
Call Trace:
 alloc_slab_obj_exts+0x1b5/0x370
 __alloc_tagging_slab_alloc_hook+0x9f/0x2d0
 __kmalloc_cache_noprof+0x1c4/0x5c0
 __set_page_owner+0x10d/0x1c0
 post_alloc_hook+0x84/0xf0
 get_page_from_freelist+0x73b/0x1380
 __alloc_frozen_pages_noprof+0x110/0x2c0
 alloc_pages_mpol+0x44/0x140
 alloc_slab_page+0xac/0x150
 allocate_slab+0x78/0x3a0
 ___slab_alloc+0x76b/0xed0
 __slab_alloc.constprop.0+0x5a/0xb0

Add the allow_spin check in alloc_slab_obj_exts() to eliminate the above
warning.

Fixes: af92793e ("slab: Introduce kmalloc_nolock() and kfree_nolock().")
Signed-off-by: Ran Xiaokai <ran.xiaokai@zte.com.cn>
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/r/20250930063831.782815-1-ranxiaokai627@163.com


Signed-off-by: Vlastimil Babka <vbabka@suse.cz>