unix: fix Fstatat by using fillStat_t on linux/mips64x

The stat structure on linux/mips64x differ between C library and the kernel,
as described in the stat(2) man page.

Fstat, Lstat and Stat on linux/mips64x already converts the stat structure
using a fillStat_t function, very similar to __xstat_conv in GLIBC.
Doing the same for Fstatat before calling SYS_NEWFSTATAT fixes the
"Fstatat: returned stat does not match Stat/Lstat" error in TestFstatat.

Fixes golang/go#29401

Change-Id: I0b2a7b274acc3c7c9fc7ae2afe722dd6225da383
Reviewed-on: https://go-review.googlesource.com/c/155747
Reviewed-by: Ian Lance Taylor <[email protected]>
Reviewed-by: Tobias Klauser <[email protected]>

Author: anthonyfok

unix/syscall_linux_mips64x.go

@@ -12,7 +12,6 @@ package unix
 //sys	EpollWait(epfd int, events []EpollEvent, msec int) (n int, err error)
 //sys	Fadvise(fd int, offset int64, length int64, advice int) (err error) = SYS_FADVISE64
 //sys	Fchown(fd int, uid int, gid int) (err error)
-//sys	Fstatat(dirfd int, path string, stat *Stat_t, flags int) (err error) = SYS_NEWFSTATAT
 //sys	Fstatfs(fd int, buf *Statfs_t) (err error)
 //sys	Ftruncate(fd int, length int64) (err error)
 //sysnb	Getegid() (egid int)
@@ -148,6 +147,7 @@ type stat_t struct {
 }
 
 //sys	fstat(fd int, st *stat_t) (err error)
+//sys	fstatat(dirfd int, path string, st *stat_t, flags int) (err error) = SYS_NEWFSTATAT
 //sys	lstat(path string, st *stat_t) (err error)
 //sys	stat(path string, st *stat_t) (err error)
 
@@ -158,6 +158,13 @@ func Fstat(fd int, s *Stat_t) (err error) {
 	return
 }
 
+func Fstatat(dirfd int, path string, s *Stat_t, flags int) (err error) {
+	st := &stat_t{}
+	err = fstatat(dirfd, path, st, flags)
+	fillStat_t(s, st)
+	return
+}
+
 func Lstat(path string, s *Stat_t) (err error) {
 	st := &stat_t{}
 	err = lstat(path, st)