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

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: fce96cf04430 ("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