lsm: secctx provider check on release

Verify that the LSM releasing the secctx is the LSM that
allocated it. This was not necessary when only one LSM could
create a secctx, but once there can be more than one it is.

Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
[PM: subject tweak]
Signed-off-by: Paul Moore <paul@paul-moore.com>