linux-user: zero fstat buffer to initialize nsec fields

The fstat implementation does not initialize the nanosecond fields in the
stat buffer; this caused funny values to turn up there, preventing, for
instance, cp -p from preserving timestamps because utimensat rejected
the out-of-bounds nanosecond values. Resetting the entire structure
to zero fixes that.

Signed-off-by: Ulrich Hecht <uli@suse.de>
Signed-off-by: Riku Voipio <riku.voipio@iki.fi>
