vfs: Add some logging to the core users of the fs_context log

Add some logging to the core users of the fs_context log so that
information can be extracted from them as to the reason for failure.

Signed-off-by: David Howells <[e-mail]>
Signed-off-by: Al Viro <[e-mail]>

fs/super.c

···
 	struct super_block *sb;
 	int error;
 
-	if (fc->fs_type->fs_flags & FS_REQUIRES_DEV && !fc->source)
+	if (fc->fs_type->fs_flags & FS_REQUIRES_DEV && !fc->source) {
+		errorf(fc, "Filesystem requires source device");
 		return -ENOENT;
+	}
 
 	if (fc->root)
 		return -EBUSY;

kernel/cgroup/cgroup-v1.c

···
 
 #include <trace/events/cgroup.h>
 
-#define cg_invalf(fc, fmt, ...) ({ pr_err(fmt, ## __VA_ARGS__); -EINVAL; })
+#define cg_invalf(fc, fmt, ...) invalf(fc, fmt, ## __VA_ARGS__)
 
 /*
  * pidlists linger the following amount before being destroyed.  The goal