fdio: allow NULL for fstatat_allow_noent stbuf

fdio: allow NULL for fstatat_allow_noent stbuf

Often, the caller doesn't actually care about the details of the stat
struct itself, but just whether the entry exists or not. It does work
to just pass `NULL` directly to glibc in a quick test, but given that
the argument is tagged as `__nonnull` and that the documentation does
not explicitly specify this is supported, let's do this safely.

Origin: upstream (submodule libglnx), 2017.13, commit:5362f6bc3ff3e30f379e767b203d15c9e56d6f08

Gbp-Pq: Topic 2017.13
Gbp-Pq: Name fdio-allow-NULL-for-fstatat_allow_noent-stbuf.patch