idpf: Acquire the lock before accessing the xn->salt

The transaction salt was being accessed before acquiring the
idpf_vc_xn_lock when idpf has to forward the virtchnl reply.

Fixes: 34c21fa8 ("idpf: implement virtchnl transaction manager")
Signed-off-by: Manoj Vishwanathan <manojvishy@google.com>
Signed-off-by: David Decotigny <decot@google.com>
Signed-off-by: Brian Vazquez <brianvv@google.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Pavan Kumar Linga <pavan.kumar.linga@intel.com>
Tested-by: Krishneil Singh <krishneil.k.singh@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>