usbip: Fix misuse of strncpy()

usbip: Fix misuse of strncpy()

Bug-Debian: https://bugs.debian.org/897802
Forwarded: https://marc.info/?l=linux-usb&m=153213915806258

gcc 8 reports:

usbip_device_driver.c: In function ‘read_usb_vudc_device’:
usbip_device_driver.c:106:2: error: ‘strncpy’ specified bound 256 equals destination size [-Werror=stringop-truncation]
  strncpy(dev->path, path, SYSFS_PATH_MAX);
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
usbip_device_driver.c:125:2: error: ‘strncpy’ specified bound 32 equals destination size [-Werror=stringop-truncation]
  strncpy(dev->busid, name, SYSFS_BUS_ID_SIZE);
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

I'm not convinced it makes sense to truncate the copied strings here,
but since we're already doing so let's ensure they're still null-
terminated.  We can't easily use strlcpy() here, so use snprintf().

usbip_common.c has the same problem.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Gbp-Pq: Topic bugfix/all
Gbp-Pq: Name usbip-fix-misuse-of-strncpy.patch