Description: netlink: Trim skb to alloc size to avoid MSG_TRUNC
References:
 https://source.android.com/security/bulletin/pixel/2020-03-01
 https://lore.kernel.org/stable/20190222160330.34237-1-salyzyn@android.com/
Notes:
Bugs:
upstream: released (4.3-rc7) [db65a3aaf29ecce2e34271d52e8d2336b97bd9fe]
4.19-upstream-stable: N/A "Fixed before branching point"
4.9-upstream-stable: N/A "Fixed before branching point"
3.16-upstream-stable: released (3.16.35) [9a11693d3def10330247d13f2db043d185002b8f]
sid: released (4.2.5-1)
4.19-buster-security: N/A "Fixed before branching point"
4.9-stretch-security: N/A "Fixed before branching point"
3.16-jessie-security: released (3.16.7-ckt20-1)