cmdlib._ReleaseLock: Do nothing if no locks are owned

The locking library doesn't like it when “release()” is called on
a lockset or lock which isn't held by the current thread. Instead
of modifying the library, which could have other side-effects,
this rather simple change avoids errors when a LU simply tries to
release all locks, even when it doesn't own any at a certain level.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>