libtpmtss: Protect TPM 2.0 context by mutex

libtpmtss: Protect TPM 2.0 context by mutex

Each private key object created to access a key residing in a TPM 2.0
creates a context structure used for communication with the TSS.
When multiple IKE SAs are established at the same time and using the
same private key, it is possible to make concurrent calls to the
TSS with the same context which results in multiple threads writing
to the same place in memory causing undefined behaviour.

Fix this by protecting calls to the TSS with a mutex unique for
each TPM 2.0 context object.