tpm2: fix dangling loaded session after tpm2_createpolicy (#484)

Clevis cleans the context regurarly from transient objects by invoking
`tpm2_flushcontext -t`, but there is one case which Clevis omits. The
`tpm2_createpolicy` command loads new session and this needs to be cleaned
by a call to `tpm2_flushcontext -l`, see [1].

This is usually not an issue when resource manager is used (i.e. with
`/dev/tpmrm0` device), but when raw TPM device `/dev/tpm0` or software TPM
(swtpm) without resource manager is used, the usage of clevis ends with
the following error:

```
WARNING:esys:src/tss2-esys/api/Esys_StartAuthSession.c:390:Esys_StartAuthSession_Finish() Received TPM Error
ERROR:esys:src/tss2-esys/api/Esys_StartAuthSession.c:136:Esys_StartAuthSession() Esys Finish ErrorCode (0x00000903)
ERROR: Esys_StartAuthSession(0x903) - tpm:warn(2.0): out of memory for session contexts
```

[1] tpm2-software/tpm2-tools#3427

Signed-off-by: Oldřich Jedlička <[email protected]>

src/pins/tpm2/clevis-encrypt-tpm2

@@ -223,6 +223,7 @@ if [ -n "$pcr_ids" ]; then
         exit 1
     fi
     tpm2_flushcontext -t
+    tpm2_flushcontext -l
 
     policy_options+=(-L "$TMP/pcr.policy")
 else