trace: fix potenial dangerous pointer

The bdi_dev_name() returns a char [64], and
the __entry->name is a char [32].

It maybe dangerous to TP_printk("%s", __entry->name)
after the strncpy().

CC: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20201124165205.GA23937@rlk
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Hui Su <sh_def@163.com>
Signed-off-by: Jan Kara <jack@suse.cz>