target: change sprintf to snprintf in transport_dump_vpd_ident

"buf" is 128 characters and "vpd->device_identifier" is 256.  It makes
the static checkers complain.

Also bump VPD_TMP_BUF_SIZE to match INQUIRY_VPD_DEVICE_IDENTIFIER_LEN.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>