exofs: Cap on the memcpy() size

This data comes from the device, so probably it's fairly trustworthy but
it makes the static checkers happy if we check it.

[Boaz]
  the system_id_len is zero, if not present, or always OSD_SYSTEMID_LEN.
  So always copy OSD_SYSTEMID_LEN bytes.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>