Revert "mm: check that mm is still valid in madvise()"

This reverts commit 4ba52630fceea53254630ac23798c4f1891041cf.

Reason for revert: <CVE-2020-29372 doest not affect kernel 5.4>

Change-Id: I82161fafcdec4363503a9dc016ae9a7c0ada947f
Reviewed-on: https://cos-review.googlesource.com/c/third_party/kernel/+/10663
Reviewed-by: Robert Kolchmeyer <rkolchmeyer@google.com>
Reviewed-by: Vaibhav Rustagi <vaibhavrustagi@google.com>
Tested-by: Saied Kazemi <saied@google.com>