[PATCH] mkfs: add discard support

[Christoph Hellwig]

Adding the discard helper to the platform headers now causes the build
to fail for some external users of the xfs headers because off64_t doesn't
exist without the large file configure magic. E.g. for xfstests:

checking xfs/xfs.h usability... no
checking xfs/xfs.h presence... yes
configure: WARNING: xfs/xfs.h: present but cannot be compiled
configure: WARNING: xfs/xfs.h:     check for missing prerequisite headers?
configure: WARNING: xfs/xfs.h: see the Autoconf documentation
configure: WARNING: xfs/xfs.h:     section "Present But Cannot Be Compiled"
configure: WARNING: xfs/xfs.h: proceeding with the compiler's result
checking for xfs/xfs.h... no

commenting out the discard helper fixes this.  We might be able to just
switch to a int64_t or similar instead, even if that's a bit ugly.