write_untracked_extension: use FLEX_ALLOC helper

We perform unchecked additions when computing the size of a
"struct ondisk_untracked_cache". This is unlikely to have an
integer overflow in practice, but we'd like to avoid this
dangerous pattern to make further audits easier.

Note that there's one subtlety here, though.  We protect
ourselves against a NULL exclude_per_dir entry in our
source, and avoid calling strlen() on it, keeping "len" at
0. But later, we unconditionally memcpy "len + 1" bytes to
get the trailing NUL byte. If we did have a NULL
exclude_per_dir, we would read from bogus memory.

As it turns out, though, we always create this field
pointing to a string literal, so there's no bug. We can just
get rid of the pointless extra conditional.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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