io: fix ftw ABI on MIPS n64

io: fix ftw ABI on MIPS n64

On MIPS n64 off_t is same as off64_t, but struct stat is not same as
struct stat64 (very peculiar but see the "as tempting as it..." comment
in linux/mips/kernel_stat.h).  As the ftw/ftw64 callback accepts a
pointer to a function who accepts struct stat/stat64, for MIPS n64 we
must use different implementations for ftw and ftw64.

Thus for testing if ftw64 can be aliased to ftw, we should check
XSTAT_IS_XSTAT64 instead of __OFF_T_MATCHES_OFF64_T.

This resolves the io/tst-ftw-lnk failure observed on MIPS n64.

Link: https://sourceware.org/glibc/wiki/Testing/Tests/io/tst-ftw-lnk
Signed-off-by: Xi Ruoyao <xry111@xry111.site>
Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>