Revert "mm: protect mm_rb tree with a rwlock"

This reverts commit 9db216b5.

Bug: 128240262
Change-Id: I210789054d394b929de6d9444040f1d2aac14917
Signed-off-by: Minchan Kim <minchan@google.com>
[dereference23: Forward port to msm-5.4]
Signed-off-by: Alexander Winkowski <dereference23@outlook.com>