Filter out redundant ioctl entries early

Filter out redundant ioctl entries early

For two ioctl entries with the same code, if one's name is a prefix
to another's name, keep the entry with a shorter name.  Filter out
redundant ioctl entries at ioctlsort stage so that distributed
ioctlent.h.in files will be already filtered.

* linux/ioctlsort.c (is_not_prefix): New function.
(main): Use it.
* linux/ioctlent-filter.awk: Remove.
* Makefile.am (EXTRA_DIST): Remove linux/ioctlent-filter.awk.
($(ioctlent_h)): Don't use linux/ioctlent-filter.awk.