locking: Fix race condition in LockSet

This patch fixes a race condition when acquiring all locks in
a LockSet instance. The list of lock names needs to be sorted
to guarantee a consistent locking order, but the names were not
sorted when acquiring all locks in the set.

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