LSM: fix return value check in safesetid_init_securityfs()

In case of error, the function securityfs_create_dir() returns ERR_PTR()
and never returns NULL. The NULL test in the return value check should
be replaced with IS_ERR().

Fixes: aeca4e2c ("LSM: add SafeSetID module that gates setid calls")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: James Morris <james.morris@microsoft.com>