syscall: simplify code using unsafe.Slice

Updates #54854

Change-Id: Ibaf4eea14a6259cdbca79e9e95db1602966f18e6
Reviewed-on: https://go-review.googlesource.com/c/go/+/428176
Reviewed-by: Bryan Mills <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: cuiweixie

src/syscall/syscall_darwin.go

@@ -310,12 +310,7 @@ func Getdirentries(fd int, buf []byte, basep *uintptr) (n int, err error) {
 			break
 		}
 		// Copy entry into return buffer.
-		s := struct {
-			ptr unsafe.Pointer
-			siz int
-			cap int
-		}{ptr: unsafe.Pointer(&entry), siz: reclen, cap: reclen}
-		copy(buf, *(*[]byte)(unsafe.Pointer(&s)))
+		copy(buf, unsafe.Slice((*byte)(unsafe.Pointer(&entry)), reclen))
 		buf = buf[reclen:]
 		n += reclen
 		cnt++