Revert "Fix build mmap"

This reverts commit c9d251aef9e3e37b1a890fa1b5fc79aa5fc8b8a6.
Revert "mm: don't pointlessly use BUG_ON() for sanity check"

This reverts commit 4b670daf73bf8e0a817c2721e3305c19527fc389.
Revert "mm: per-thread vma caching"

This reverts commit 243087936a2795fca18f16bda8a780e5cfb1075b.