igb: Fix strncpy calls to be safe per source code review tools

This fix changes the remaining calls to strncpy that have not yet
been changed to use the "sizeof(buf) - 1" syntax rather than just
a number for buffer size.

Signed-off-by: Carolyn Wyborny <carolyn.wyborny@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>