Description: binfmt_elf: switch to new creds when switching to new mm
References:
 https://www.openwall.com/lists/oss-security/2019/04/03/4
 https://www.openwall.com/lists/oss-security/2019/04/03/4/1
Notes:
 bwh> I'm not sure whether this is worth the risk to backport - there may
 bwh> be subtle ordering/locking issues.
Bugs:
upstream: released (4.8-rc5) [9f834ec18defc369d73ccf9e87a2790bfa05bf46]
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 (3.16.66) [1e06334afa7199cc86c6c4830b71a7c1ea8e7901]
sid: released (4.8.5-1)
4.9-stretch-security: N/A "Fixed before branching point"
3.16-jessie-security: released (3.16.68-1)