Call SyscallConn() instead of Fd() to use file's fd

[os.File.SyscallConn] is the safer alternative to [os.File.Fd], without the pitfalls mentioned in Fd's doc comment. The SyscallConn method is implemented on all platforms except Plan 9, where it returns [syscall.EPLAN9], which matches [errors.ErrUnsupported] and goes to the fallback path.

reader.go

@@ -234,7 +234,7 @@ func Open(file string, options ...ReaderOption) (*Reader, error) {
 return nil, errors.New("file too large")
 }
 
-data, err := mmap(int(mapFile.Fd()), size)
+data, err := openMmap(mapFile, size)
 if err != nil {
 if errors.Is(err, errors.ErrUnsupported) {
 data, err = openFallback(mapFile, size)
@@ -257,6 +257,21 @@ func Open(file string, options ...ReaderOption) (*Reader, error) {
 return reader, nil
 }
 
+func openMmap(f *os.File, size int) (data []byte, err error) {
+rawConn, err := f.SyscallConn()
+if err != nil {
+return nil, err
+}
+
+if cerr := rawConn.Control(func(fd uintptr) {
+data, err = mmap(int(fd), size)
+}); cerr != nil {
+return nil, cerr
+}
+
+return data, err
+}
+
 func openFallback(f *os.File, size int) (data []byte, err error) {
 data = make([]byte, size)
 _, err = io.ReadFull(f, data)