CVE-2026-46318: Revert "mm/hugetlbfs: update hugetlbfs to use mmap_prepare"

In the Linux kernel, the following vulnerability has been resolved:

Revert "mm/hugetlbfs: update hugetlbfs to use mmapprepare"

This reverts commit ea52cb24cd3f ("mm/hugetlbfs: update hugetlbfs to use mmapprepare") with conflict resolution to account for changes in commit ea52cb24cd3f ("mm/hugetlbfs: update hugetlbfs to use mmapprepare").

The patch incorrectly handled hugetlb VMA lock allocation at the mmapprepare stage, where a failed allocation occurring after mmapprepare is called might result in the lock leaking.

There is no risk of a merge causing a similar issues, as VMADONTEXPANDBIT is set for hugetlb mappings.

As a first step in addressing this issue, simply revert the change so we can rework how we do this having corrected the underlying issues.

We maintain the VMA flags changes as best we can, accounting for the fact that we were working with a VMA descriptor previously and propagating like-for-like changes for this.

Note that we invoke vmasetflags() and do not call vmastartwrite() as vmflagsset() does. This is OK as it's being done in an .mmap hook where the VMA is not yet linked into the tree so nobody else can be accessing it.