misc: fix gcc-7 sprintf warnings -Wformat-overflow

misc: fix gcc-7 sprintf warnings -Wformat-overflow

../login-utils/last.c: In function ‘main’:
../login-utils/last.c:624:23: warning: ‘%s’ directive writing up to 31 bytes into a region of size 27 [-Wformat-overflow=]
   sprintf(path, "/dev/%s", ut->ut_line);
                       ^~   ~~
../login-utils/last.c:624:3: note: ‘sprintf’ output between 6 and 37 bytes into a destination of size 32
   sprintf(path, "/dev/%s", ut->ut_line);

../libblkid/src/devname.c: In function 'probe_one':
../libblkid/src/devname.c:166:29: warning: '%s' directive writing up to 255 bytes into a region of size 245 [-Wformat-overflow=]
   sprintf(path, "/sys/block/%s/slaves", de->d_name);
                             ^~
../libblkid/src/devname.c:166:3: note: 'sprintf' output between 19 and 274 bytes into a destination of size 256
   sprintf(path, "/sys/block/%s/slaves", de->d_name);
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>