commit b817541f1cfd38e4b81257b3215e276ea9d0fc61 introduced statx with
a fallback using fstatat, but failed to fill in stx_rdev_major/minor
and stx_attributes[_mask]. the rdev omission has been addressed
separately. rather than explicitly zeroing the attributes and their
mask, pre-fill the entire structure with zeros. this will also cover
the padding adjacent to stx_mode, in case it's ever used in the
future.

explicit zeroing of stx_btime is removed since, with this change, it
will already be pre-zeroed. as an aside, zeroing it was not strictly
necessary, since STATX_BASIC_STATS does not include STATX_BTIME and
thus does not indicate any validity for it.

The current implementation of the statx function fails to set the
values of stx->stx_rdev_major and stx->stx_rdev_minor if the statx
syscall fails with ENOSYS and thus the statx function has to fall back
on fstatat-based emulation.