security: check for kstrdup() failure in lsm_append()

lsm_append() should return -ENOMEM if memory allocation failed.

Fixes: d69dece5

 ("LSM: Add /sys/kernel/security/lsm")
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: James Morris <james.morris@microsoft.com>