misc: fix gcc-7 snprintf warnings -Wformat-truncation

misc: fix gcc-7 snprintf warnings -Wformat-truncation

../lib/loopdev.c: In function 'loopcxt_next_from_sysfs':
../lib/loopdev.c:545:32: warning: '/loop/backing_file' directive output may be truncated writing 18 bytes into a region of size between 1 and 256 [-Wformat-truncation=]
   snprintf(name, sizeof(name), "%s/loop/backing_file", d->d_name);
                                ^~~~~~~~~~~~~~~~~~~~~~
../lib/loopdev.c:545:3: note: 'snprintf' output between 19 and 274 bytes into a destination of size 256
   snprintf(name, sizeof(name), "%s/loop/backing_file", d->d_name);
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

../lib/sysfs.c: In function 'sysfs_is_partition_dirent':
../lib/sysfs.c:343:31: warning: '/start' directive output may be truncated writing 6 bytes into a region of size between 1 and 256 [-Wformat-truncation=]
  snprintf(path, sizeof(path), "%s/start", d->d_name);
                               ^~~~~~~~~~
../lib/sysfs.c:343:2: note: 'snprintf' output between 7 and 262 bytes into a destination of size 256
  snprintf(path, sizeof(path), "%s/start", d->d_name);
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../lib/sysfs.c: In function 'sysfs_partno_to_devno':
../lib/sysfs.c:372:32: warning: '/partition' directive output may be truncated writing 10 bytes into a region of size between 1 and 256 [-Wformat-truncation=]
   snprintf(path, sizeof(path), "%s/partition", d->d_name);
                                ^~~~~~~~~~~~~~
../lib/sysfs.c:372:3: note: 'snprintf' output between 11 and 266 bytes into a destination of size 256
   snprintf(path, sizeof(path), "%s/partition", d->d_name);
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../lib/sysfs.c:377:33: warning: '/dev' directive output may be truncated writing 4 bytes into a region of size between 1 and 256 [-Wformat-truncation=]
    snprintf(path, sizeof(path), "%s/dev", d->d_name);
                                 ^~~~~~~~
../lib/sysfs.c:377:4: note: 'snprintf' output between 5 and 260 bytes into a destination of size 256
    snprintf(path, sizeof(path), "%s/dev", d->d_name);
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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