cred: allow get_cred() and put_cred() to be given NULL.

It is common practice for helpers like this to silently,
accept a NULL pointer.
get_rpccred() and put_rpccred() used by NFS act this way
and using the same interface will ease the conversion
for NFS, and simplify the resulting code.
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

include/linux/cred.h

@@ -237,7 +237,7 @@ static inline struct cred *get_new_cred(struct cred *cred)
  * @cred: The credentials to reference
  *
  * Get a reference on the specified set of credentials.  The caller must
- * release the reference.
+ * release the reference.  If %NULL is passed, it is returned with no action.
  *
  * This is used to deal with a committed set of credentials.  Although the
  * pointer is const, this will temporarily discard the const and increment the
@@ -248,6 +248,8 @@ static inline struct cred *get_new_cred(struct cred *cred)
 static inline const struct cred *get_cred(const struct cred *cred)
 {
 	struct cred *nonconst_cred = (struct cred *) cred;
+	if (!cred)
+		return cred;
 	validate_creds(cred);
 	return get_new_cred(nonconst_cred);
 }
@@ -268,7 +270,7 @@ static inline const struct cred *get_cred_rcu(const struct cred *cred)
  * @cred: The credentials to release
  *
  * Release a reference to a set of credentials, deleting them when the last ref
- * is released.
+ * is released.  If %NULL is passed, nothing is done.
  *
  * This takes a const pointer to a set of credentials because the credentials
  * on task_struct are attached by const pointers to prevent accidental
@@ -278,9 +280,11 @@ static inline void put_cred(const struct cred *_cred)
 {
 	struct cred *cred = (struct cred *) _cred;
 
+	if (cred) {
 		validate_creds(cred);
 		if (atomic_dec_and_test(&(cred)->usage))
 			__put_cred(cred);
+	}
 }
 
 /**