config: use -Wno-format-truncation globally

-Wformat-truncation looks for places where the return code of snprintf()
is unchecked and the provided buffer might be too short. This is based
on a heuristic that can change between compiler versions.

It has been seen to get this wrong in ddt_object_name(), leading to
DDT_NAMELEN being increased somewhat arbitrarily.

There's no good reason to have this warning enabled, so here we disable
it everywhere. Truncation may be undesirable, but snprintf() is
guaranteed to emit a trailing null, so at worst we get a short string,
not a buffer overrun.

Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Sponsored-by: https://despairlabs.com/sponsor/
Closes #15908