integrity: add validity checks for 'path' parameter

This patch adds validity checks for 'path' parameter and
makes it const.

Signed-off-by: Dmitry Kasatkin <d.kasatkin@samsung.com>
Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
diff --git a/security/integrity/digsig.c b/security/integrity/digsig.c
index 5e3bd72..36fb6b5 100644
--- a/security/integrity/digsig.c
+++ b/security/integrity/digsig.c
@@ -85,7 +85,7 @@
 	return err;
 }
 
-int __init integrity_load_x509(const unsigned int id, char *path)
+int __init integrity_load_x509(const unsigned int id, const char *path)
 {
 	key_ref_t key;
 	char *data;