nvmet: fixup buffer overrun in nvmet_subsys_attr_serial()

The serial number is copied into the buffer via memcpy_and_pad()
with the length NVMET_SN_MAX_SIZE. So when printing out we also
need to take just that length as anything beyond that will be
uninitialized.

Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>