fdio: Be const-correct for struct stat
We don't modify this struct (if non-NULL), so it can be const. In
particular, this is helpful if calling glnx_file_copy_at() from
nftw() to implement the equivalent of cp -a --reflink=auto
.
We don't modify this struct (if non-NULL), so it can be const. In
particular, this is helpful if calling glnx_file_copy_at() from
nftw() to implement the equivalent of cp -a --reflink=auto
.