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

[ Upstream commit 87ea58433208d17295e200d56be5e2a4fe4ce7d6 ]

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>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>