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

This reverts commit ba1366d26de21d436faafbc41be3a3648c2dccc5.

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

Change-Id: If1ac00023d8a4c4f7d45168738e4e3154c8197a8
Reviewed-on: https://cos-review.googlesource.com/c/third_party/kernel/+/10660
Reviewed-by: Robert Kolchmeyer <rkolchmeyer@google.com>
Reviewed-by: Vaibhav Rustagi <vaibhavrustagi@google.com>
Tested-by: Saied Kazemi <saied@google.com>
Main-Branch-Verified: Saied Kazemi <saied@google.com>
1 file changed