IMA: Check IMA policy flag

process_buffer_measurement() may be called prior to IMA being
initialized (for instance, when the IMA hook is called when
a key is added to the .builtin_trusted_keys keyring), which
would result in a kernel panic.

This patch adds the check in process_buffer_measurement()
to return immediately if IMA is not initialized yet.

Signed-off-by: Lakshmi Ramasubramanian <nramas@linux.microsoft.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

security/integrity/ima/ima_main.c

@@ -655,6 +655,9 @@ void process_buffer_measurement(const void *buf, int size,
 	int action = 0;
 	u32 secid;
 
+	if (!ima_policy_flag)
+		return;
+
 	/*
 	 * Both LSM hooks and auxilary based buffer measurements are
 	 * based on policy.  To avoid code duplication, differentiate