papi: Use CMSG_SPACE for sizing ancillary buffer space

CMSG_LEN provides the length required to store an ancillary item, but it
does not consider padding. RFC 3542 requires portable applications to
use CMSG_SPACE(), this is documented in the python API:

https://docs.python.org/3/library/socket.html#socket.CMSG_LEN

Type: improvement
Change-Id: I0eab470585f2f39bea38639db95d6b0e047d2cff
Signed-off-by: Tom Jones <thj@freebsd.org>

src/vpp-api/python/vpp_papi/vpp_stats.py

@@ -54,7 +54,7 @@ import re
 def recv_fd(sock):
     """Get file descriptor for memory map"""
     fds = array.array("i")  # Array of ints
-    _, ancdata, _, _ = sock.recvmsg(0, socket.CMSG_LEN(4))
+    _, ancdata, _, _ = sock.recvmsg(0, socket.CMSG_SPACE(4))
     for cmsg_level, cmsg_type, cmsg_data in ancdata:
         if cmsg_level == socket.SOL_SOCKET and cmsg_type == socket.SCM_RIGHTS:
             fds.frombytes(cmsg_data[: len(cmsg_data) - (len(cmsg_data) % fds.itemsize)])