split: avoid apparent infloop when splitting /dev/zero w/-n on the Hurd

split: avoid apparent infloop when splitting /dev/zero w/-n on the Hurd

* src/split.c (main): Use stat.st_size only for regular files.
Samuel Thibault reported in http://bugs.gnu.org/11424 that the
/dev/zero-splitting tests would appear to infloop on GNU/Hurd,
because /dev/zero's st_size is LONG_MAX.  It was only a problem
when using the --number (-n) option.
* NEWS (Bug fixes): Mention it.
This bug was introduced with the --number option, via
commit v8.7-25-gbe10739