linux(4): Fix control message size calculation

To determine the size in bytes needed to hold a control message
and its contents of length len, CMSG_SPACE should be used.

Reviewed by:
Differential Revision: https://reviews.freebsd.org/D41224
MFC after: 1 week

(cherry picked from commit 67116c69052d27dbd6d6c066104850d4d53f0041)