kexec-tools: Fix possible out-of-bounds access in ifdown

Fix a possible out-of-bounds access in function ifdown():

ifdown.c: In function 'ifdown':
ifdown.c:56:4: warning: 'strncpy' specified bound 16 equals destination size [-Wstringop-truncation]
   56 |    strncpy(ifr.ifr_name, ifp->if_name, IFNAMSIZ);

Signed-off-by: Helge Deller <deller@gmx.de>
Signed-off-by: Simon Horman <horms@verge.net.au>