mm/mseal: update VMA end correctly on merge

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

mm/mseal: update VMA end correctly on merge

Previously we stored the end of the current VMA in curr_end, and then upon

iterating to the next VMA updated curr_start to curr_end to advance to the

next VMA.

However, this doesn't take into account the fact that a VMA might be

updated due to a merge by vma_modify_flags(), which can result in curr_end

being stale and thus, upon setting curr_start to curr_end, ending up with

an incorrect curr_start on the next iteration.

Resolve the issue by setting curr_end to vma->vm_end unconditionally to

ensure this value remains updated should this occur.

While we're here, eliminate this entire class of bug by simply setting

const curr_[start/end] to be clamped to the input range and VMAs, which

also happens to simplify the logic.