pythongh-129005: Avoid copy in _pyio.FileIO.readall

`os.read` allocated and filled a buffer by calling `read(2)`, than that
data was copied into the user provied buffer. Read directly into the
caller's buffer instead by using `os.readv`. `self.read()` was doing the
closed and readable checks so move those into `readinto`

Author: cmaloney

Lib/_pyio.py

@@ -1694,11 +1694,12 @@ def readall(self):
 
     def readinto(self, b):
         """Same as RawIOBase.readinto()."""
-        m = memoryview(b).cast('B')
-        data = self.read(len(m))
-        n = len(data)
-        m[:n] = data
-        return n
+        self._checkClosed()
+        self._checkReadable()
+        try:
+            return os.readv(self._fd, (b, ))
+        except BlockingIOError:
+            return None
 
     def write(self, b):
         """Write bytes b to file, return number written.