Description: userns: also map extents in the reverse map to kernel IDs
References:
 https://bugs.chromium.org/p/project-zero/issues/detail?id=1712
Notes:
 jmm> Introduced in 6397fac4915ab3002dc15aae751455da1a852f25 (4.15-rc1)
Bugs:
upstream: released (4.20-rc2) [d2f007dbe7e4c9583eea6eb04d60001e85c6f1bd]
4.9-upstream-stable: N/A "Vulnerable code introduced later in 4.15-rc1"
3.16-upstream-stable: N/A "Vulnerable code introduced later in 4.15-rc1"
sid: released (4.18.20-1)
4.9-stretch-security: N/A "Vulnerable code not present"
3.16-jessie-security: N/A "Vulnerable code not present"