virt: sevguest: Fix return value check in alloc_shared_pages()

If alloc_pages() fails, it returns a NULL pointer. Replace the wrong
IS_ERR() check with the proper NULL pointer check.

Fixes: fce96cf0 ("virt: Add SEV-SNP guest driver")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Brijesh Singh <brijesh.singh@amd.com>
Link: https://lore.kernel.org/r/20220411111213.1477853-1-yangyingliang@huawei.com

drivers/virt/coco/sevguest/sevguest.c

@@ -581,7 +581,7 @@ static void *alloc_shared_pages(size_t sz)
 	int ret;
 
 	page = alloc_pages(GFP_KERNEL_ACCOUNT, get_order(sz));
-	if (IS_ERR(page))
+	if (!page)
 		return NULL;
 
 	ret = set_memory_decrypted((unsigned long)page_address(page), npages);