ListXAttr called with an empty buffer returns the current size of
the list but does not touch the buffer (see man 2 listxattr).

Fixes the xfstests generic/020 crash.
Fixes https://github.com/hanwen/go-fuse/issues/337 .

Add test
Co-authored-by: Han-Wen Nienhuys <hanwen@google.com>
Change-Id: I94c94bc56fdf506d2eb924b246336b787fa2976a