JFS: Avoid segfault when dirty inodes are written on readonly mount

Writes to a device may cause a ->write_inode to be called during a read-only
mount.  JFS needs to check for NULL log in jfs_flush_journal.

fs/jfs/jfs_logmgr.c

@@ -1415,6 +1415,10 @@ void jfs_flush_journal(struct jfs_log *log, int wait)
 	int i;
 	struct tblock *target;
 
+	if (!log)
+		/* jfs_write_inode may call us during read-only mount */
+		return;
+
 	jfs_info("jfs_flush_journal: log:0x%p wait=%d", log, wait);
 
 	LOGGC_LOCK(log);