mm/ksm: add missing IS_ERR_OR_NULL check for stable_tree_search()

mm/ksm: add missing IS_ERR_OR_NULL check for stable_tree_search()

The stable_tree_search() maybe return -EBUSY if the stable node's page is
being migrated or nullptr, we need to check kfolio with IS_ERR_OR_NULL()
before dereference it.

To mitigate this, add IS_ERR_OR_NULL check for stable_tree_search().

Link: https://lkml.kernel.org/r/20241024032300.2501949-1-cuigaosheng1@huawei.com
Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Cc: Alex Shi <alexs@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>