lsm,nfs: fix memory leak of lsm_context

commit b530104f ("lsm: lsm_context in security_dentry_init_security")
did not preserve the lsm id for subsequent release calls, which results
in a memory leak. Fix it by saving the lsm id in the nfs4_label and
providing it on the subsequent release call.

Fixes: b530104f ("lsm: lsm_context in security_dentry_init_security")
Signed-off-by: Stephen Smalley <stephen.smalley.work@gmail.com>
Acked-by: Paul Moore <paul@paul-moore.com>
Acked-by: Casey Schaufler <casey@schaufler-ca.com>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>