Fix CVE-2020-29372 in 4.19 and 5.4

Hi Greg,

To fix CVE-2020-29372 in COS kernel versions 4.19 and 5.4, we cherry-picked the commit "mm: check that mm is still valid in madvise()" (bc0c4d1e176e) that Jens introduced in kernel version 5.7.0 into our kernel sources. The commit is small and the cherry-pick was successful for both COS kernels versions.

Because COS 4.19 and 5.4 kernels track 4.19.y and 5.4.y respectively, can you please cherry-pick the commit to those stable branches?

Thanks,

--Saied

COS kernel source: https://cos.googlesource.com/third_party/kernel